I think you need to throw a putter different than an approach disc. With an approach, I think you're throwing with still quite a bit of spin, and with a relatively straight trajectory. With a putter, you're more tossing the disc up and letting it float in (thus the larger rim), with decreased spin when compared to an approach. If you are throwing a putter like you would throw an approach, this would explain your difficulty.
That having been said, I played with a guy last weekend that threw every putt hard at the chains - I mean it didn't change elevation at all. I believe the conventional wisdom is against this, as if you miss, you've got another tough putt (among other reasons); but the guy didn't miss once from inside 30' (and kicked my a$$ for the round). Am I thinking about putting wrong (floating it in), or is this guy just an anomaly?