Did this start happening after a recent update?
I had this exact thing happen to me after an update, it was like only being able to play limit in a no limit game.
The fix was actually quite simple (if your case is the same as mine).
Try deleting the shortcut to 888 on the task bar then either log in directly from the icon on the desktop or pin a new shortcut to the task bar.
The reason it works when you first install the new update is you aren't using the icon on the task bar, it fires up automatically after the update, but as soon as you use the old task bar icon you get that weird glitch that only allows you to min bet or min raise, but pinning a new icon to the task bar after the new download works, as does using the desktop icon directly.😎