Cranks but won't start

I just recently bought a 2006 TDI VW Beetle. Sometimes it cranks and cranks and no start. I put a new battery and now running some biodiesel through it. That seemed to help until it happened again this morning. It will eventually start after trying a few times. Anyone have any idea what it could be??

have you checked you lift pump?

Clogged fuel filter?

Is the check engine light illuminated?

the upshot of what aja8888 and I are saying is - have you checked that you have fuel at your rail? Easiest way to check quickly is to check your fuel filter - see if it has any fuel in it. If there isn't, then there doesn't appear to be any fuel from the lift pump.

Have you checked the links in my signature?

The check engine light did come on the one time it happened but not since. I changed the fuel filter as soon as the problem happened. However I don't know what a lift pump is. Is that expensive? Hard to replace?

According to the links in my signature, the lift pump is in the fuel tank and pumps fuel to the filter and to the engine. It is easy to listen for the pump, just remember that just because you hear the pump doesn't mean that it's pumping. DanG has a great thread about testing it and I linked it in my signature.