If you cannot see the chat of other players at the poker tables, you may have accidentally modified some of the chat visibility options in your account.
If the chat of only one player is blocked, to unblock it:
If the chat of all players (including you) is blocked, to unblock it, click on:
If the chat of observers is blocked, follow the same path as above, and on step 5, click on Show observer messages.
For dealer's chat, click on: