My machine had similar symptoms. See my comment below. GL!
I had a similar issue others have described wherein the machine (MacBook Pro 13" Unibody Mid 2010) would recognize either of the new IFixIt sticks in either position when paired with either of the OEM sticks (8 permutations tested, LOL), but not both new sticks paired together. OSX, SMC, EFI and associated firmware appeared up-to-date. Purchased a different set of 2x8GB sticks from a competitor and they worked like a charm. Noticeable performance increase from factory 4GB and from 10GB hybrid. To their credit, IFixIt accepted my return without question despite being well past their standard 30-day policy. Very happy with them, and will continue to give them my business.
FWIW, new (working) RAM appears identical, spec-wise: 2x8GB PC3-8500 1066MHz. Timings are printed 7-7-7-20. Couldn’t find timing noted on IFixIt RAM. I wouldn’t think a difference would matter… maybe the IFixIt memory wasn’t compatible with the controller in some way. Anyway, a potential solution to try.