This is a known issue that many users are experiencing. It works for your account because you're the connected account owner, but fails for outside users.
Things to try:
1. Check your first message node - Make sure it's set to "Send as COMMENT REPLY" not "Within 24-hour Window." This is the most common cause.
2. Add "Set Instagram Opt In" action - Put an Action Node → Set Instagram Opt In right after your trigger node. Some users report this fixes it.
3. Refresh your Facebook connection - Disconnect and reconnect Instagram in ManyChat. Wait 1-2 hours before testing again.
4. Test with a completely fresh account - Comment triggers only fire once per user per post. If you've tested before with that account, try a different one.
5. Check Message Requests folder - DMs from automations often go to the recipient's "Message Requests" folder, not their main inbox. They won't see quick reply buttons there either - use solid buttons instead.
Unfortunately this is a known Meta-side issue that ManyChat has escalated. It affects many accounts randomly and there's no guaranteed fix yet.
Feel free to DM me if you need more help troubleshooting, happy to take a closer look.