Tracks won't sync in a loop

Hello,

I have two cdjs 2000 nexus. If one Track is running and I want to mix in another using a loop there is about a fifty fifty chance that the tracks sync. Sometimes it works sometimes not. Although the tracks very often run away from each other if one of them is in a loop. (in this case im not using sync because its not working) For example there is the first track running and a second track in a loop. The second track in this case will go faster (not changing the bpm) and the tracks will not be beat matched anymore. This is very annoying because it happens wherever I play. So its not a matter of the hardware but the software.

ill be really grateful for any help

Best regards Henk