What I did
I bought a handgun through the mail through GunBroker, too. Have you contacted them and asked if they can be any help? I doubt it, but it might be worth asking if your FFL had any history of complaint.
My experience was that I arranged with the receiving FFL to send his license to me and to the seller (there never was a question about the seller's license).
There was a glitch because of a widespread power outage that took my receiving FFL off line and out of phone communication for 2-3 days, but that was quickly fixed when he got back in communication.
So, the only advice I can give you is similar to wow6599's. Get copies of both FFL licenses and confront your guy who has your rifle. Before you go, program BATFE on your speed dial.
If it gets that far, expect to never do business with that FFL holder ever again (even if you were willing).
Your local Better Business Bureau might be able to help, too, especially if the FFL is a member. Your your local gun community might be in a position to apply "incentive". But be careful with that threat.
If you noise it about what this guy is doing, you open yourself up to libel suit, but truth is a defense. Make sure you have proof that everything you say is true. True or not, proof is your responsibility in a case like that. I had a problem like that and the seller (furniture) came to my First Sergeant and asked him to tell me I was risking a libel suit for what I said about his bed. I had talked to my 1st Sgt about it beforehand. He told the seller, "I have seen the bed and he's right." Settlement followed swiftly.
Lesson: Have ALL your ducks in a row before taking any steps. ALL of them.
Theft may be too heavy a charge (depending on the laws in the jurisdiction), but misappropriation seems the minimum I would call it.
I always assume good will, so when making your next confrontation, give your FFL the benefit of any available (or unavailable) doubt and invite him (by language, gesture and attitude) to explain why he does not release your rifle to you. [He MAY actually have a legitimate reason-be prepared for that.] You can always escalate later. Eventually hitting that speed dial. (It would help if you had the name of a BATFE agent with whom you have had prior contact. This, just in case my story about my 1st Sgt missed the mark.)
Good luck
Lost Sheep
p.s. To triple up on the advice: If you are in control of the timing (which, in this case, you are) NEVER enter a firefight without your backup (Plan B AND plan C if you can get one) already in place.