I don't use anything for this so I have no personal experience. But looking at Google Play it does say it's compatible with my non-Maxx Razr so it should be with your Maxx as well. Their website looks pretty useless as far as support (no manual, faq, support forum) but I'm wondering if you've tried emailing the developer. Perhaps there's a bug they are not aware of?
Alternatively, I noticed
Just Shake Free has 4,870 4 star ratings and
Shake Call has 1,960 - compared to only 232 for Shake to Answer. They might be worth "a shake."
