The most common reason a test fails is that the comment came from the same account that published the post. If you comment your keyword from the same Instagram or Facebook account that posted the video, the automation will not trigger, so it looks broken when it is working correctly.
How to test it properly
Use a second account or a friend's account to comment your keyword. The DM should arrive on that account.
Turn on Instagram DM access first
In the Instagram app: open Settings, go to Messages and Story Replies, select Message Requests, find Allow Access to Messages, and turn the permission ON. This step is required or nothing will send.
Tutorial: https://youtu.be/7T-CNexRhQA
If you have tested from a separate account with DM access on and it is still not sending, email [email protected] with a link to the post you tested on.