You are signed in on the app I assume? Such that when you use the left most top icon to start a new flight, it presents a dropdown list of all your aircraft on the new flight page? And on the myflightbook.com website, under Preferences, under Flight Entry, you have " [Maybe the app has those fields always enabled?]
My suggestion would be that you provide a Starting Hobbs and/or Engine Start number at startup. I've always done that (occasionally missing one or the other), and never had a problem with MFB auto-detecting my takeoffs, landings and airports visited. Except once, maybe twice. (Maybe because I failed to fill in either of those aforementioned fields?) It may need some indication like that to know that this is a real flight, because I know after finishing a flight and clicking on Add it puts the app back on the new flight page. So I'm wondering if it needs some kind of entry in one or more of the fields to tell that there really is a flight going to happen. Otherwise, it would've logged my driving home from the airport after each flight. (Unless it triggers not only by ground speed, but also by altitude -- but I think not.)
I know Comments and Route don't need to be filled before flight because I almost always do the comment section after the flight, and the route is filled in by MFB.
You can test whether your GPS is functioning OK (I know you said above that it's giving a good signal, but maybe there's an issue when it's moving?) by installing one of many many GPS apps out there. You can also test that by bringing up iFly and driving around (it has a trigger speed -- I don't remember if that's a user setting; but I know freeway speed definitely tells iFly you're "flying"). Once you determine that the GPS is not the issue I'd suggest driving around with MFB running. I've never tried it, but it seems like it should work.
I hope you get it working, it's a pretty good product. If you do, be sure you contribute something to the creator.