TF2 and The Orange Box Question

Why is it that when I played TF2 I only had the ability to pick only one class?

There were times where i could only pick the spy or the sniper and i would press A to pick the scout and nothing would happen.

Can someone explain why i only had one class available while playing TF2?

Valve updated the game so that the host can block some classes and turn of critical hits. The only thing you can do is join a different lobby.