Each game has a minimum and a maximum bet per round. Those limits can change from one game to another and also depend on your account currency.
In general, games that can pay a higher maximum win, as a multiple of the bet, allow a lower bet per round. That is why one slot may let you bet more than another.
How to see a game's limit
Open the game and go to the bet panel.
Raise the bet with the + button. When the button turns grey, you have reached that game's maximum bet.
The amount that counts is the total bet per round, not the intermediate values in the panel.
Betting more per round does not change the game's odds: it only makes each round cost more and your balance move faster. If you want to control your play, you can ask support for help.