How do you know that the problem is due to the one shots not behaving as they should? Are you arriving at that conclusion by watching the performance of the machine or have you programmed traps to verify that the one shots never pulsed? One shots are notoriously difficult to troubleshoot because you can't count on seeing them pulse on the monitor.
One shots require an additional table in memory to keep track of the previous state of the bit. I don't know the details of this PLCs' architecture, but if the problem is in hardware, you'll need to replace the module where the transition table is located. Before replacing any hardware, I'd make sure the problem is not in the application program.