Woodz428 might be on to something with the lifter preload.
Did you put new valves in the heads? If so, then the vavle stems could be longer than the stock valves, or.......
is it possibe that when the valves and seats were ground down, to much material was removed and allowing the valves to seat deeper in the head, thus, in effect, making the valve stems "longer" on the top of the head. This in turn would load the non-adjustable lifters sooner.
Just some thoughts
