Well, I do not have an easy answer to your problem. Need to step back and look at what is known right now. Lifters adjusted correctly, this means the pre-load on the lifter plunger is correct. Without beating this to death, we will assume it is. I hate that word, 'assume'! Given that, what can cause a 'gap' in the valve train that would give you the 'ticking' noise? Checking valve train geometry by marking the valve stems will verify correct pushrod length and that the rocker is not sliding off the valve stem. What else? Extreme cases, rocker studs pulling out, adjusting nut backing off, rocker arms badly worn, valve stem tips badly worn? Cam lobe to lifter contact area worn badly. The other more common cause, the lifters are bleeding down. Dirt in the lifter in just the right place will cause this. I have seen RTV lodged in there causing the bleed down.
One more thing you can try, as others have suggested, adjusting with the engine running. Personally, I have not done this for years, but I have in the past. Once the lifter preload is set, prior to engine run, it should never change or change very little. Not enough to cause a ticking noise. Guys who race with hydraulic lifters like to set the preload at the very minimum, this gives max valve lift. So, if you want to try this be aware it will get messy very quickly. You used to be able to buy clips that go over the pushrod ends to redirect the oil squirting out of the end of the pushrod. I have also modified a valve cover buy cutting a 1" wide slot the length of the cover so you can access the adjustment nut with the cover installed.
Without any attempt to slow oil flow, warm the engine up first, this will lower oil pressure at idle. With valve cover removed, expect to lose a quart of oil, running down the engine block, exhaust, ect... on to the floor. Depending on how fast you are adjusting. Basically back off the nut until it clicks loudly, then tighten until it stops, and go 1/4 to 1/2 turn. Given your scenario, it may stop ticking for a couple cycles then become noisey again. I suppose you could tighten all the lifter plunger movement out so the 'bad' lifter can't bleed down, but you run the risk of a valve not closing. This would be a very fine line to adjust to. Others may have more suggestions, but that's all I got.
Keep us up-dated as to what you find.