Problems with SYNC in RB 5.4.3

I got some problems with the sync function in Rekordbox. If I have two tracks playing I just activate the "BPM Sync" on each deck. The master tempo is set. 

But sometimes the second track is just not in time and the beatgrids are not set correctly. Only the speed is adjusted to the master tempo deck - but the beatgrids are not moved automatically in sync. 

I also checked the quantize option. That makes no difference. 

Sometimes it seems that this problems just appears while having set a loop. Before looping the sync functions works fine. Having set a loop in the same track there is no function when I active "BPM Sync".

Can anybody help?

I am on Mac OSX 10.14.3 and use a DDJ RR. But the problem is also without the controller plugged in there.