Can be recreated::
While using towny you have claimed plots and claimed land. This bug happens to occur when a piston is attempting to move a block from one to the other.
[Piston]----|[Block]
And
[Piston]|[Block]|{Plot End}
The piston will not fire into a claimed plot, or in my case, out of a claimed plot. It may have something to do with the perms we have set as we use plots as security against box and door access from outsiders. The perms theory has not been tested but when I removed said interfering plot the piston fired off just fine.