In multiplayer, transporters will often not detect a player walking onto them. Additionally, the transporter computer and visual effect continue to indicate that the transporter is active for a very long time, but it does not seem to be functional during this period. (However, the difficulties with getting a transporter to transport you in multiplayer in the first place makes this hard to test.)
Pressing Tab-F6 will reset the transporter when it gets stuck, so it is presumably a client/server synch problem.