When your scope was mounted on the rifle with your previous mounts did it "lean" to the left then also? Reason I ask is to determine if the problem lies with the new mount or with the grooved dovetails that are cut into your receiver. Ideally the scope should line up directly centered on the receiver top, but of course we don't live in a perfect world and sometimes when they cut those dovetail grooves into the receiver they do so a little off center. Or,,, it's possible the dovetail grooves are centered and straight but the new mount is a little "off". I have one GAMO rifle that has a raised scope rail and the rail is just a little, very little, off center which makes my scope angle ever so slightly to the right. I'ts such a little bit of an angle you have to look close to even notice it, but my scope setup works fantastic so I really don't care. Is your scope leaning a tiny little bit, or is it really noticeable? Are you sure you have the one piece mount properly installed and the mount base properly placed into the dovetail grooves on both sides? It can be easy to overlook not having the mount base properly aligned inside the grooves. I am by no means any kind of expert when it comes to anything airgun related, so I'm sure some of the more knowledgeable guys on the forum may be able to advise you better, but what I've mentioned are just some things that came to my mind. Also, another topic but,,,,, make sure you thoroughly clean the dovetail grooves, receiver top, and scope mount elements with denatured alcohol to properly clean them before you permanently mount your scope. And use blue loc-tite on the screws.