ModelCheck not returning same results

I have ModelCheck configured to issue an error and prevent check-in if a part does not have a parameter set to a known value or does not exist. If I create a new part and delete the parameter, ModelCheck functions as expected, and the check-in is blocked. If I have an existing part that got checked-in before I enabled the check for the parameter, it allows the check-in to go through and issues no error or warning for that file. The existing file was loaded into Windchill with the old ModelCheck rules, before I tightened them for this parameter. When running ModelCheck interactively, it still does not generate an error. MC_Verified is set to YEs from the prior check-in and we have set the value for ModelCheck hours to 100,000.

Has anyone else seen this behavior before?