Interval Based PM Increasing Intervals

We have a piece of equipment that has an interval based PM.  The interval is 30 days.  The intervals have been increasing each time there is a PM scheduled.  First schedule to second was 41 days.  Second schedule to third schedule was 56 days.   Fourth schedule to fifth was 77 days.  Our expectation is these should be scheduled 30 days from the prior completion date. 

Any idea what is wrong?

Thanks,

Jeff

EOne 9.2

TR 9.2