When a booking is being made on a specific item, the fact is recorded in a file named LockSess.tps.
I'm making two bookings right now, one in NetBookIt, one in BookIt. Here's what is in LockSess:
The first record with the IP address as User is NetBookIt. The second record is BookIt.
If another user tries to book the same, BookIt checks LockSess and finds that I have priority on those items.
When I close the booking form the relevant record is deleted.
But if I don't close the form, that record stays there, blocking other users. BookIt should ignore that block after 10 minutes, assuming that I have forgotten to close the window.
Delete LockSess.tps from the BookIt folder and you won't be blocked.