Multiple tracks are not in sync

Why is it that when I record multiple tracks one after the other the new tracks do not line up with the old tracks. They are out of sync.

A little information will helps give you some possible answers:
What version of Ntrack are you using?
Did it work befor some change to the computer?
Internal sound card or an add on card?
What sound protocol are you using - mme, WDM, ASIO?
What operating system?