What you can do is take the end of a hammer handle (grabbing the head of the hammer) and push down on the valve side of each rocker arm, and hold down on it hard for about 10 seconds.
Then release pressure on the handle and see if that arm is loose or not. If you find one that's loose, that's the trouble maker, you can try replacing just the hydraulic element on that particular arm.
Do this one at a time until you find the bad one or ones. Be sure to check if all the cup-shaped washers are there on top of the valve stem.