Some methods of recording rides/workouts, in particular Garmin and TrainerRoad have slightly different XML tags. TrainerRoad, for instance, adds "ns3:" (less the quotes) to many of the tags in the TCX file that gets downloaded.
The result is that Ascent ignores some of the data in a TCX file imported from these services, in particular, the power data from TrainerRoad. Currently, I am removing "ns3:" manually before importing to Ascent (see this thread where I discuss doing so).
Would be great if Ascent recognized these tags as legitimate without having to edit them first.