Gday Pete,
I had the saem problem..
There is a setting for 'clear current track memory' or the like it was showing that it was at 90% so it would get full and keep deleting the previous route I had walked on the map and starting again.
Jump on google/youtube and you will find a tutorial on tracks which will cover this and more.
Another possibility is the SD card is close to full and cannot supply enough space to that feature for logging current tracks.
Best of luck. Master reset wont help.