remiss

Remiss describes someone who has been careless or negligent in fulfilling a duty or responsibility.

If you would be remiss not to do something, you would be failing in your obligation by neglecting it: I would be remiss if I didn't thank everyone who helped.

The word carries a tone of mild formal reproach rather than harsh accusation, often appearing in polite acknowledgments of oversight or in gentle criticism of someone's inattention to their responsibilities.