Beat sync did work now it isnt - FLX4 and Spotify

Hi all, really hoping you can help before the Xmas break.  I have had my FLX4 for 5 weeks.  Running Rekorbox on the latest version and utilising Spotify through it.  Firs 2 weeks everything worked perfectly however the last 3 weeks this is not the case.

Beat sync will not work at all on any track and when manually mixing by ear the beat match slips as well.

I have unistalled rekorbox, put everything back to default, deleted my account and re-registered and even gone back and installed older versions of Rokordbox to see if this will work but no luck.

Used my spotify account through a friends rekorbox and DDJ400's and worked perfect.

I am not looking for comments about mixing by ear and buy your tracks, I brought this set-up to use at home and intro my kid to DJ'ing and just looking for it to work correctly for us and upcoming Xmas house parties with friends.

Any help would be greatly appreciated as spent 3 weeks trying to sort this, researching and trying every tip online possible.

Many thanks in advance.