An automation is not replying

The eight causes, in the order they usually turn out to be.

Work down this list. One of these is nearly always it.

It is a draft, or switched off. An automation needs to be live and switched on.
Another automation answered. Only one replies to each comment; yours will show as skipped. See Only one automation replies.
Exact mode. Exact requires the whole comment to be your keyword. Use Contains instead.
An exclude keyword matched somewhere in the comment.
Wrong post. The automation watches specific posts and this was not one of them.
Out of quota. Check Usage and limits. Only the exhausted allowance stops.
Instagram refused it. Usually a closed messaging window, or a comment that already had its one DM.
Your Instagram account is disconnected. You would have been emailed. See Staying connected.

It replied publicly but sent no DM

The follow gate is on and they have not confirmed they follow you. This is by far the most common reason.
Your message allowance is exhausted while comment replies remain.
That comment already received its one private reply.

A catch-all automation answers anything nothing more specific claimed. Add an exclude keyword, narrow it to specific posts, or block that contact.