We discovered today thanks to an upload from euant that if you use the Ordnance Survey's iOS app to record your track1 then each track point in the resulting GPX track has the position of the point but lacks either the time the point was recorded or the elevation according to the GPS. This was stopping the GPX mapping application from loading the track (at all!).
We have now upgrade the app so that tracks like this can be uploaded. The lack of elevation data doesn't actually matter much as we calculate the point's elevation (with rather more accuracy than a GPS can manage) from the point's position but without time we can't give much data in the "Your Track" data box as this screen shot, based on euant's track, illustrates.
If you've seen euant's post which originally started us looking into this issue you'll already know that there are related effects on hill and walk log pages (which may have affected some other recent walk reports too in a similar way) so our next step is to investigate and fix those issues.
We'll keep you posted.