Google Play Music Forgets Where I Left Off

Dear collective brain

The Google Play Music Free app on my latest Android phone (Samsung Galaxy Core Prime + OS 5.1.1) mostly forgets where I left off the previous time I paused my song (actually 1 hour lecture). My previous phone which had OS 4.something remembered just fine but this one only remembers if I only pause for a short time.

If I use another app for a little while and come back to play I have to find the place in the track again.
If I close the app (ie swiping it from the running apps screen where it has images of each running app) and open the app again the same thing happens.

It is "as if" each time it gets removed from RAM it loses its place.

I'm sure this problem has been dealt with many times. I've done some searching but I can't tap into the collective to work out what to search for. Tried things like "Google play app forgets where I'm up to" and a few other searches and keep getting hits about the x best player apps etc. No trouble shooting on the issue.

So if you could either
1) tell me if you know what's causing the issue or
2) advise me on how to search for this problem (best search terms) I would appreciate it.

Thank you.