trying to understand a it actually works

i have figured out most of how the program works and i love it, but one thing that has always given me a problem is when i am trying to take an older track and lay down grid markers to straighten out the tempo....now everything looks good, but when i use the sync function with two tracks beatmixing it does not stay in sync. the only thing that i have found that kind of works is continuously hitting the sync button to catch it back up. do i not have something set right, is there something that i am missing?