We ran into something like this earlier this week with a customer, but it was with live chats. However, the same may hold true for tickets: If you have business hours set, make sure the time zone you have set for your Agents matches the time zone set for your installation. Otherwise, some agents will get tickets and chats at different times. The time zone settings for agents is on the Options tab when editing an Agent.
You may also want to make sure there are no Max Tickets set for these agents. Or if you do use the Max Tickets setting, you may need to adjust it to accommodate ticket load.
Then, there are a few "make-sure-it's-plugged-in" type things to check: Agents are part of the group that receives the tickets, all agents are marked as Available, etc.
I hope that helps.