This could be the result of several problems so let's start with the easy ones. How much cam lift and what type of rocker arms?

If the rockers are stock type are they binding on the rocker stud before full lift? If so they wi;ll have marked the edge of the stud. Do you have enough piston to valve clearance?

Is the valve spring ibstalled height enough for the cam you have?
Check a valve at full lift and see if you can slip a .060 feeler gauge between the spring coils. Or you could try pushing down on the valve at full lift, it should be able to go at least another .050.

These three things will give you a start.