When you open it up in design mode, is the filter property set to anything that you did not set it to? If so, clear it out and re-save it. If you open it later and find that it has inherited a filter again, you then realize that something else is saving the change to the form.
This could be happening when the end-user adjusts the filter property by using the filtering tools on the toolbar.
I don't know what version of MS Access you are using yet, so it's hard to tell you more.