If you think it's one rocker only you will need to do a complete check of the components. First pull the shafts and check the 4 valves on that side with a dial indicator you can use a sharp point in the end of the push rod and line the dial shaft straight up this will check the lift on the cam. Next measure and check straightness of the push rods.

If the push rod is bent replace it and try again.
If the lift is off you can lay a flat plate on the shaft locations and measure to the center of the lifter to see if it' not pumped up.

Hopefully it's one of those two buy if it's not you will have to get the lifter out and measure the lift of the cam to see if the lobe is damaged.