Whenever I try to play poker, all tables are empty.

And if not, there is always the same person sitting there, that single player I'd rather not play with again. My very first online poker game ever (on this site and any other) I won against him against the odds, and since then he is blasting me with a stream of complaints about what a bad player I am and what dumb luck I have and such.
No, I'm not gonna name him. If you played him, you know who I mean.

Back to the topic: there are seldom enough people there when I check.
Time zones around the world might be a reason; low interest might be a reason.
Here are four ideas to make it easier to find someone to play with:
1. On your main and game pages, there could be an icon (maybe a playing card) that shows if someone is waiting for poker. So while I go through my games and move, I see it and could go play poker.
2. When you want to play poker, you set a flag, and whenever you are online, it shows your name to all others that set the flag too (similar to 'friends' functionalty - you see right away which friends are online / you see right away who wants to play poker).
3. Reduce the number of tables. Only one or two are ever used; the rest is just clutter.
4. Allow the players already on the table to deny access to specific individuums (similar to 'block' function). That would allow to play games without having 'him' show up immediately and drown you in whining.