I have experienced the same - in my case, the mobile app autotracks a couple of trips then ignores the next few trips.  Then it might track a few more.  Very frustrating.

For what it's worth to anyone experiencing one of the problems I described above (i.e. that often when I manually record a trip, the mileage shows that I travelled 0 miles), I found a work around.  In my mobile app, I open the "Mileage" module and go to the "reviewed trips."  Then I click on a trip showing 0 miles and click "edit".  I then click on the starting address and save it again.  Then do the same with the ending address.  Somehow this makes the calculated distance correct itself to show the correct distance between the points.  I can then save the whole transaction and go back to the list of reviewed trips.  It's a pain in the @SS, but at least it's better than having to re-enter the whole transaction.  A better option would just be for Quickbooks to fix this mileage module - please!