Both Dan Patterson and Kyle Balke, GISP have important points on what may be causing the unexpected behavior. It may be that you are trying to edit your shapefile outside the established methods, which is why it is not working correctly. Can you elaborate on the specific processes you are doing to make the changes to your shapefile?
It would be best to ensure the process you are using is viable, as there are some processes people use that seem like they would work but actually have negative consequences. For example, editing a shapefile attributes using Excel to change the values in the .dbf part of the shapefile - that is NOT a valid method, as it does not update the linkages between the table (.dbf) and the spatial index (.shx). However, it is easy to assume since one can open the dbf in Excel that this is OK.
Also, the link below may prove helpful to recover files that are already showing unexpected behavior (though you probably want to figure out why they are getting corrupted first).
Recovering a corrupt shapefile:
27429 - Recover a corrupt shapefile
Chris Donohue, GISP