It's already been said ...
... but do some checking with paper.
Remember also that 15mm can be 13 yards ... not OVER 13 yards.
It is quite common for shooters to find that when they shoot at very close targets, that are very blurred, the point of impact is not only low ... but will be to one side.
Some folk will say this is PA error. Some will say that it's because the scope isn't perfectly lined up with the barrel due to the mounts or the dovetails etc.
I found that I could just aim top of kill for everything from 13 yards or shorter. However the POI was not only low but to the left. If I was practising and focused my scope to 13 yards ( or whatever close distance I was shooting at ) then the POI was directly below the cross. As I put the focus back to 25 yards, then using a blurred picture the POI was ALWAYS to the left. I tried checking PA error and was convinced that my eye was central. It must have been my scope rings or dovetails. I just got used to aiming top of kill and edge of kill right and they went down.
You can try head further back or squinting the eye. All help clear the image ... until you get so old ... then you are goosed.
The best and only way is to shoot at paper at 15, 14 ... 8 yards and see where your pellets are hitting. If they are going to the side then your choice whether you try and straighten it out or just aim accordingly.
Practice your ranging on those distances ... but as mentioned ... you can get your set up so you just aim TOK for 13 yards down etc.