In A Form, Some Dates Work, Some Don't

I've got a form with several date fields (in Medium date format), with each field bound to fields in a table, where also the dates are in medium format.

I also have the form set up where I can enter dates manually, or I can double-click and pick dates from a calendar form, or I can click on an icon and pick dates from the calendar form. Each date goes through the same bunch of processes (i.e., queries and code).

My problem: Some dates work, and some dates don't. On some date fields, I can key in a date, exit the form, re-open the form, and my dates show up (since they were processed into my table.) However, some other date fields do not get processed! I enter dates into these fields, exit the form, re-open the form, and there are no dates!!

I have checked all fields in my table..... they are all date fields, of medium date format. Same for my form.

Why would some dates work just fine while others don't work? If anyone has some suggestions I could try, I would be grateful. What might I look at in my application?