Users who escape jail are okay, due to the way the jail command is designed, they can't destory or create any more blocks, use any commands, or use the R key to return to their last loaded location.
I think the issue lies in the amount of players, or the current load on the server when it comes to jailing users. Since it peforms a move command, sometimes it can "miss" and not jail them. If this is the case, perform the following command:
Code:
/move [name] 5 22 250
Which is the location of jail.
I'll take a look at the code for jail, and see if it can be optimised.