[AccessD] checkboxes aren't functioning

Checkboxes have a tri-state, on, off, and Null. You might want to check that
-- perhaps someone accidentally changed it?

Although, if I'm not mistaken, even if the control is Null, you should still
be able to change it. 

Susan H. 

how is it possible that suddenly on a form in almost al records certain
checkboxes are working but in some records they don't function (are greyed
out and can't make them "true" or "not true").
Nothing strange happened before.