Online sports betting providers advertise with celebrities, offer free bets, and grant various perks to present a tempting opportunity: winning. However, some gamblers who manage to beat the betting providers report that their bets are often rejected once they become too successful.

Dave Holmes, a sports bettor from Chicago, said that companies like BetMGM, ESPN Bet, and Caesars had started to reject his bets as he won more and more with a math-based betting strategy. He typically bets about $100 per wager. Some companies instead offered to accept only bets of 50 cents.

I would really like to get an answer to why this is happening, but they refuse to explain it," said Holmes.

Some frequent bettors search through the odds of everything from NFL and MLB games to table tennis and darts, looking for weaknesses in the prices set by bookmakers and placing advantageous bets, similar to how hedge funds look for undervalued stocks.

Bettors who have had their options limited say that the guessing game about why and when this happens is frustrating. Some have suggested that bookmakers should be required to offer the same maximum to all bettors or explain why some bettors are subject to different limits.

Earlier this year, the Massachusetts Gaming Commission held a public discussion about betting limits and raised questions about how online sports betting operators decide how much money to accept from individual customers. Several sports betting operators licensed in the state - including DraftKings, FanDuel, BetMGM, Caesars, ESPN Bet, and Fanatics - agreed to attend the meeting but did not appear, fearing that business-critical information might be disclosed publicly, according to the commission. Since then, they have agreed to discuss the matter with the commission.

The regulator is examining betting limits at a time when the other end of the sports betting business is coming under increased scrutiny: the biggest losers. Companies place big bettors in so-called VIP programs and offer them thousands of dollars in free bets and other perks, as reported by the Wall Street Journal in February. In response, U.S. Senator Richard Blumenthal (D., Conn.) sent letters to online gambling companies asking how they use game data and other marketing methods to target customers.

Sports betting providers do not report regularly on how many customers are restricted or how many are part of VIP programs. Nakisha Skinner, a member of the Massachusetts Gaming Commission, expressed concerns about transparency and fairness during a public meeting in March.

You have a situation where a customer is losing and is encouraged by an operator's outreach program and incentives," said Skinner. "What is the balance when that same customer starts to win?

Sports betting managers say that restricted bettors make up only a fraction of their clientele and are typically professional players – in contrast to most customers who bet on sports for fun. Heavy betting with promotions could lead to a customer being restricted, according to some companies.

Sports betting providers employ complex risk management and trading departments to monitor cash flow from various bets and minimize losses while offering attractive odds for customers. The companies say they need to manage their risk to operate profitably.

In land-based casinos, it has always been possible to block winning players, including in sports betting, and many successful bettors have to find ways to disguise their bets so that the casinos cannot track what they are doing. The legendary sports bettor Billy Walters employed teams of people who spread out in Las Vegas and placed bets for him.

Online gambling companies typically set a maximum amount that customers can bet for each wager offered. Some gamblers find that their personal betting limits are lower when they log in to place a bet.

Michael Holt lives in South Dakota, near the Iowa border. Since his home state has not legalized sports betting, he drives to Iowa to legally place online bets.

During "March Madness," he placed some $50 bets on basketball. He won a few of them and noticed in the following days that ESPN Bet and Caesars would only accept bets of $5 or $10 from him. So far, he hasn't noticed any limits on his bets with DraftKings or FanDuel.

Holt said he bet a total of $1,000 on the ESPN Bet app and won $1,200, resulting in a profit of $200.

The fact that they marked a player with a win of 200 dollars is pretty crazy to me," he said.

A spokesperson for DraftKings said the company offers tens of thousands of bets daily, all with market-specific betting limits. "We restrict less than 1% of players below the market limit, mainly based on betting behavior," he said.

Penn Entertainment, which operates ESPN Bet, stated in responses submitted to Massachusetts regulators that customers receive an on-screen message if a bet exceeds their approved limit, which is updated in real time, and that customers can have these limits raised. Any laws or regulations that would prevent operators from implementing such safety measures would force them to reduce their offerings, according to the company.

Holmes, the sports bettor from Chicago, researched a mathematical approach to gambling that he claims improves the odds in favor of bettors and said he has won nearly $50,000 since he started.

He said he was restricted by five companies without explanation. Originally, Holmes tried to place $100 per bet, but now depending on the company and type of bet, he cannot wager more than $10, $25, or $50.

If companies were to openly and honestly tell players directly that they have won too much and take measures to manage business risk, he said: 'I would understand that very well.'