1) Are you trying to "activate" an existing loop?
2) Import all your tracks to your collection, then it doesn't matter - you can search and it will find the track. If you don't have all your music imported, then you have to analyse any new song on import, so why not prepare everything in advance by having it imported, analysed, and beat gridded?
3) The analysis continues to improve with each version. Are you sure you have it set for the right mode? NORMAL mode is for songs with electronic percussion and DYNAMIC mode is for songs with a live drummer where the timing of the tempo may vary slightly.
4) When in SYNC, only the MASTER deck can change the tempo. If the deck that was the MASTER has stopped, the assignment of which deck is the master will change.
5) You'd need to provide a lot more information, as there are dozens of reasons why the track would lag when loading.