Hello Graham
Martin Brennan here.
The BB1 will re-shuffle the tracks on occasions and that probably explains why you hear tracks again recently.
The tracks are reshuffled whenever you make a change - add, delete, rename.
But more importantly it will reshuffle when it restarts if you turn it off and on.
As for not hearing some tracks - you would need to listen to all 1420 tracks - without turning it off and without making changes - to guarantee hearing them all.
Hope that helps
Martin