Descriptions a bit vague and i don't think there is a setting, but hopefully this helps. My trails just writes over itself after a few trips out. If you want to keep it, make a new trail each trip (memory use is high). 

Extract from manual: 

" The unit will continue to record a trail until the trail length reaches the maximum trail point setting. When the unit reaches the maximum trail point number it will automatically begin recording the trail over itself"

 Do you just run track the whole time you have the plotter on? if so, the memory will just fill up and then start overwriting. I know Lowrances' system of having the bloody thing start plotting  the moment you turn it on is a major PITA.  Hate it myself. My MO is to turn it to just the plotter screen before I shut down each time. That way , the first thing I do as I leave the ramp next time is go Menu>waypoint marks trails> trails>delete. The new trail which has just started up will be the one highlighted, so just delete it. When you need a trail start a new one. Proper management of trails can make them very useful--eg, if a trail shows a safe path between obstacles, save it to a particular colour. Then you'll know you need to keep it. When you are starting a drift, turn it on. Same for trolling, lures out and turn it on. Save it if you want to keep it. Just leaving the thing running the whole time from startup gives you a screen which looks like spaghetti vomit, and will eat up the stuff you really wanted to keep.

Pretty sure in the settings you can set how many miles you wish to save before the start of the track starts disappearing

 If the middle of the track disappears it sounds like that your GPS is loosing it's position and not recording the track.

A full memory would result only in older tracks disappearing. Where it the unit mounted?

Just a thought.

Steve