There are so many variables here, so many possibilities. It is so difficult to diagnose things this way (I've recently been on this site looking for ideas myself), moreso with a lack of detail.
What engine?
What type of valvetrain? Adjustable or non-adjustable? OEM or aftermarket?
If adjustable, have you checked valve adjustment?
If non-adjustable, are the pushrods correct length?
It could be so many things, this is just what popped into mind.