start up

Had trouble starting the Corvalis this morning.  I have both batteries on, mixture set to full rich, throttle open about 1/4".  I depress the primer for 5 seconds (although it's hard to tell for sure if it stays depressed since it doesn't appear to be a manipulator that is visibly depressed).  I turn the key, the props turn over and I can hear the starter, but the engine doesn't start.  Are there any "likely culprits" that I might check?  I didn't have this issue the other day so I don't know what could be wrong.

Thanks...

fuel selector on?

That is one thing I did not check, will keep that in mind.  I did reload plane and it started right up.  cheers...