@megger
I can usually help. But that, I mean I can almost always help someone figure out what's wrong. And, I can usually take care of the problem, if it IS fixable. Sometimes I don't even need to work on the gun.
I've worked on a lot of the RWS/Diana guns, including the barrel-cocking guns like the 36, and, I know a lot of the reasons why they would give you that wandering POI problem.
Each manufacturer tends to have their own list of mechanical reasons why things like that can happen. And, that's definitely true of the 36 and its same-brand cousins.
Often times, someone and I can have a phone conversation, and by their giving me more information, and answering some questions, we can either figure it out then and there, in 15 or 20 minutes, on the phone.
or,
based on what they tell me, I can narrow it down to just one or two things. At that point I will likely have to actually see 'the problem child' (the rifle) in person.
I CAN tell you here that trying to do it via the printed word, by text or email that is, is often just too hard. It can take a couple of hours of written messages back and forth, to do what we can do in 20 minutes or less on the phone.
I can also tell you here that a couple of the first things I suggest to anyone with that same problem are to:
1. Nicely snug-up the main stock screws on the gun. In your case, that's 3 of them: one on each side of the front of the stock, and the big one in the front of the trigger guard.
2. Try a few different ammunition brands, and stick with the same one out of the same tin or box, if possible. Then, use the the one that gives you the very-best results in your gun.
Don't bother to test outdoors on a windy day. You probably knew that one.
As far as sticking with the same ammunition, trying to force-feed any air gun a given ammunition, even if others absolutely swear by it in the same model of air gun, is often a waste of your time and energy. These air-shooters of ours tend to be very-finicky when it comes to ammunition. Even guns with 2 consecutive serial numbers often do best with 2 different ammunition brands and/or sizes.
Speaking of that, sticking with the same ammo includes using the same head size of that brand, for example "4.52 mm" (for .177 caliber), if it's written on the back of the container. Sometimes, the specific size isn't provided. That doesn't mean the ammo won't work for you.
3. I can also tell you here that one of the first things I suggest is to snug-up all the screws on the scope and scope mounts, if you're using a scope. If that doesn't help, and you've done the other things mentioned above, try switching to a different scope and mount set-up that is already giving you no such problems whatsoever on another recoiling air gun.
4. If you don't have such a scope and mounts to try, (or even if you do), if you want to try something fast and easy, do this:
If your gun has open sights, just remove the scope, and use the open sights, at even a short distance, like 15 or 20 yards. If the groups clearly-tighten-up with open sights, the problem is obviously with the scope and mount set-up you have.
Your scope may be touted as being unbreakable. But, if you think about it, that can never be true. No matter the price, the reputation, or the warranty, anything mechanical can break.
Please check for a PM.
BTW, most-important:
I have a couple of must-finish items here right now. But, if you want to talk, I can probably spare the 20 minutes or so within the next couple of days.
Chances are, your 36 is quite fixable. Thanks.
HTH.