well even if you have yet to find an obvious issues,
least your logically testing and narrowing the potential possibility
ok, basics, the oil enters the lifter from block oil passages under pump pressure while the lifters on the cams base circle,
with near zero valve spring pressure on the lifter seat,
the hydraulic oil pressure lifts the push rod seat taking up the valve train clearance.
as the cam rotates the lifter moving up off the cams base circle causes the lifters push rod seat to momentarily move down compressing the trapped oil volume , as the push rod seat is compressed,down into the lifter body,
forcing oil trapped in the lifter up through the push rod,, the trapped , and now further pressurized oil is there for two different requirements,
first it lubes and cools the valve train, as it exits the rocker/push rods and flows over the rocker and valve springs,and second it quiets the valve train by removing the clearance, with a cushioned floating push rod seat,
that clearance is required for the valve train to function.
but the clearance results in a ticking sound like solid lifters produce,
adding a hydraulic supported push rod seat quiets the valve train,
and without the hydraulically supported push rod seat,the valve train requires periodic clearance adjustments ,
the hydraulic supported push rod seat is fully compressed, by the cam lobe forcing the lifter up against the push rod and the valve springs resistance,
by the time the cam lobe reaches peak lift ,
but at lower rpms all the oil is forced up and out of the lifter well before the lifter is near full lift on the cam lobe,
now it is possible that at extremely high rpms the lifter bleed down rate does not have sufficient time to vent all the trapped oil,
this is commonly thought to result in valve float,
but thats not likely, the inertial loads on the lifter and valve train not being completely controlled by the valve spring resistance,
and ability to seat the valve firmly and keep it seated as the lift returns to the cam lobe base circle,
and the lifter loosing firm contact with the cam lobe and or bouncing,
as it re-seats is far more commonly the fault.
most of the GIVE or clearance or movement in a hydraulic lifter push rod seat is the result of pressurized oil flowing into and lifting the push rod seat, without the pressurized oil entering the lifter body,from the sides of the lifter body from the oil passages in the block, manually forcing the push rod seat down into the lifter body is basically a one shot deal that generally only going to compress in a minimal .060 -.08 once until oil pressure forces the seat back up.
http://garage.grumpysperformance.com/index.php?threads/correctly-adjusting-valves.196/#post-90731