2 things I can think of, but they depend on what view you are in when you
close the form and/or how the form is being closed:

1) If you are not in design view when changing the form, perhaps the form
"Allow Design Changes" property has been changed from "All Views" to "Design
View Only"

2) If DoCmd.Close is being used to close the form, perhaps the Save argument
has been changed to acSaveNo. (This argument refers to the saving of Form
Design changes NOT changes to data on the form). acSavePrompt is the
default.

You may have code that executes DoCmd.SetWarnings False, without
subsequently executing DoCmd.SetWarnings True to turn the warnings back on.