As far as I understand, you need an AR15 Weaver Mount like this one for Weaver rings to fit. I am not able to fit my Weaver rings onto my 1913 Picatinny rail.
Check this page and let us know which set of rings you're using.
http://www.weaveroptics.com/rings_bases/rings/rings.aspx
Weaver states that even with the Weaver type base built into an AR15 will accept Weaver rings but the highest ring (1" See-Thru #49513) still isn't high enough to clear the front sight.
I think you may have that the other way around. And they still don't grab very well.