I recently bought an upper receiver from POF USA and a brand new lower receiver from Rock River.  When I charge the weapon, it feels like the hammer is rubbing too hard onto the bolt carrier and the bolt will actually stick in the rear position and will not ride forward on its own unless I jar it a little bit.  I have used my upper on other lowers before without a problem, so I'm thinking the problem is the spring on the hammer is too strong maybe because it is new.  Has anyone heard of this problem and if so, knows how to fix it?

Something might be out of spec... Out of spec Hammer, hammer pin hole or an over rated spring like you said...

Are you using a new bolt carrier too? Could be that also...

ETA: If you are positive that upper works with other lowers then it shouldn't have anything to do with the carrier key - gas tube alignment then...

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Is this an AR-15 bolt where the hammer gets caught on the firing pin?

After reading your post again, check the buffer tube and buffer retainer... One of those parts could be causing the bolt carrier to bind and drag...

Well that would be the problem if he were using an M16 hammer. An RRA LPK uses an AR-15 hammer, or at least it did as of a few months back. If you're not using major brand lower parts, well then Randall probably just found your problem.

Separate your upper and lower and simulate your bolt running through your upper and locking into your barrel extension. Do you feel any resistance from the bolt, the gas tube, the carrier key, etc?  If not, it's probably a problem in your lower.

There is a lot of stuff that happens as that hammer rides past the end of the firing pin since that's the beginning of when an AR goes into battery. Any one of these things can cause resistance.

Just a long shot, but is it possible your bolt catch is rubbing against something?

Well, either way the last RRA LPK I saw had a hammer without a notch. Randall's probably right saying it's an AR-15 hammer.