You don't say what caliber you are shooting, but assuming that it is reasonable for the distance, then your issue is probably that the bases are mis-aligned and causing you to use too much of the adjustment in your scope just to zero at 100yds. You can replace your rings with some Burris Z-rings (get the optional assymetrical inserts) and they will allow you to dial in up to ~30MOA of additional elevation with one set of inserts and up to ~40 moa if you really need it and get two sets of inserts.
I should disclose that I know nothing about your scope, but I've never seen a scope that doesn't have 20 MOA of vertical travel ... if this scope just doesn't have any real travel, then you could still use the Z-rings, but you obviously won't be able to tune both 100 and 600 yds.
Hope this helps ...
Saands
ps ... adding shims to the bases is not a really good idea if you have 2-piece bases, mostly because you will end up with the bases being out of alignment and you will stress the scope tube when you force it to comply with the newly misaligned rings/bases.