So we know its compression, how can you be at 15psi cranking?
Hole in piston.
Valves not closing (you've already eliminated this as a possible cause however).
Rings/Lands demolished.
Crack in the head/block.
Bad (seriously bad) Headgasket.
Spark plug not actually seated (possibly crossthreaded).
Cams are way out of time.
So start with the simple things first, its probably one of those.
Like I suggested to another cam issue member, a leakdown test will quickly reveal where the problem lies.
If your valve clearances are in spec, then your valves are closing, at least well enough to register more than 15 psi. Cam Timing or not seated sparkplug are the two things I'd be checking first.