Nearly zeroed - Nikon Prostaff 3-9x40 Rifle Scope, 1in Tube

I bought for my Gamo .177 air rifle for better viewing than the basic scope supplied with the gun - interestingly, it was easy to zero the basic scope but desperate to zero the Nikon. Why? I think because the Mounts are too low therefore the much larger Objective end of the scope is too close to the barrel - I have sent for higher sitting Mounts and hope this deals with the issue. To get close to zero I have had to spin the adjusters on the scope far too much.
Any other ideas would be welcome - I have given 4 stars because the clarity is excellent - with new mounts I hope to upgrade to 5 stars.