I've had few players sign up via my link at other Casino, but it's hard to keeping them playing unless there's something we can give to them.
Such actions are also quite difficult for a casino. The players then come back briefly to get a "Welcome Back" bonus, for example, gamble it away and are then gone again. I think it is unrealistic that many players who have already stopped playing on a platform will continue to play there after a longer break.
In general, I think referral actions make much more sense here: New players offer a much greater chance that they will remain active on the platform for longer.
It's not that hard. My friend had a casino review website and he was getting great profit from it. Not everyone but good percentage of people who were registering via his affiliate link were active players. When you say that you have to give them something in order to keep them, it doesn't directly means the money, really. For some people, the give can be the fact that you helped them found a casino with great UI, promotions, privacy and all other services.
Seems your customers were only the people who were seeking for some free bucks and that's all, they may registered for a single time to claim good promotion.
Btw there is a single reason when player may return to casino: There was a case of Bitcoin halving some years ago, I mean BCH. A lot of people had some amount of bitcoins left on casinos and when the casinos supported the adoption of BCH, they deposited the equivalent amount of BCH on their accounts. It was a free money, some would gamble and some would withdraw.