Sounds like you put open chamber heads on already low compression piston. It takes 80 to 90psi of compression just to fire an engine. If you have hydrolic lifters and you set them not pumped up they maybe to tight and trying to pump up as oil pressure comes up while your turning over the engine, causing the valve to stay open.
Sorry about the edit, but I wanted to ad that if you timing chain is not on the right centerline, and is retarded to much you can loose 50 to 60 psi.