when I add the next one it makes it disappear!
I'm not an Edge user, but ... are you sure?
Are you checking each Track by using 'view on map' or some such - this
does suppress any neighbouring Tracks, on my Etrex. The only way to check that both (all) are visible is to scroll the 'live' map from your current location to where you expect the join to be, and zoom in to a normal working zoom level. Tedious, but you only really have to check that 2 are working, if they are, all the rest will be OK as well.