I think the OP has tightened the rocker down and the lifter piston has moved down past zero lash..... Then when the oil pressurizes the lifter it floats the valves.
With the intake manifold off I always look at the hydrolic lifter's top as I am turning the adjuster down, lifting the pushrod up and down as do this. If the top starts to compress you are at zero lash dry. Go 1/4 turn and LOCK that sucker down, do the next 15.
Fire it up and run the cam in at 2000 RPM for the 20 minutes. Then while still hot and lifters are pumped up, run the lash.