Having inadequate valve spring pressure can cause this. They could be allowing the lifters to pump up.
Be sure to check the lifter preload and geometry. Being that the valve train is non adjustable, the push rod length is critical.
Changes to the following will effect the p-rod length requirement:
Camshaft base circle diameter
Pushrod length due to wear or replacement
Different thickness head gaskets
Valve seat height changes from seat replacement or from valve seat grinding/cutting done during valve jobs
Grinding the tip of the valve stem when doing a valve job or different length valves
Lifter pushrod seat height due to different manufacturer
Change in rocker arm type or design, or ratio