Element Helix 6-24X50 FFP

I have an Element Helix 6-24X50 FFP scope. I mounted it on my S510 rifle and find it is shooting 7 inches low at 20 yards. It is almost out of “up” by the time it is zeroed. I have no problem putting pellet on pellet at this range, so the rifle isn’t tossing them around. The rifle has had a Sightron and a Meopta scope on it, both I was able to adjust for 20 yard zero easily with a few clicks. I am using BKL rings on it, if the rings are swapped end for end it makes no difference, still shoots quite low. Could this be a scope issue? I did the mirror style zero check, and the scope appears to be centered at zero elevation and zero windage. Seems like I should be able to zero and click elevation out to 100 yds on this scope like I can on a similar 22 cal rifle with a Discovery 6-24 on it.
 
I use SportsMatch vertically adjustable rings on my S410/510 series rifles to take care of this issue.
I only find the situation odd because of the 3 scopes I have used on this rifle, it is the only one that has had this issue. The Athlon on my S400 was within a couple clicks of zero at 20 yds. I haven’t had to use an adjustable ring set on a pcp for normal ranges. I’ve only used adjustable rings on a Walther springer for droop.