Why won't my car start?

I have a 2004 Jetta. It is a manual. It
is turning over but not starting. It has
a full tank of gas. I had the same
problem a couple weeks ago. The
next morning the car started so I
took it to the mechanic. He said he
couldn't find anything wrong with it.
He checked the fuel pressure and it
was reading fine. He went ahead and
replaced the spark plugs because
they were shot. He also replaced the
air filter. Now a couple weeks later
it's doing the same thing.