Andrew,
If you the rebuilder is nearby and you have any type of dialogue with him, I certainly would take it back and have him give it a post-op inspection...something isn't right, obviously.
The only other issue might be junk that got into the needle/seat area when you installed the 'working' carburetor. This would prevent the bowl from filling properly, although you indicate the accelerator pump squirts when actuated.
The base must be gasketed and tight. You also indicate this has been done properly. If you did your part, then the rebuilt carburetor should work. If it doesn't, the rebuilder should have no problems taking a look at it. Make sure all your bases are covered before you take it back to him!
Stuart
