It may be that little compression (looks to me like .125-.25 at most) paired with immediate high speed is not enough to impart much velocity on the disc. Since you appear to have the materials handy, I suggest you look into a
Spectrum (Team 3847) esque shooter. You would be able to sub the BAG motors for 2 CIMS, the smaller wheels for your bigger wheels, and also I suggest looking into slowing down the first wheel that the disc contacts.