With the Wait() added, as Jeremy suggested, the solenoid should operate the way you expect.
The solenoids we get in the Kit won't supply the fast, sudden pressure you need to launch a t-shirt though. There are different solutions for how to do the valve that other teams have used.
T-shirt launchers have been talked about a lot on Chief Delphi, so check out some of those threads for how other teams have done it: