I have had several very tight Cox engines in the past, but it's usually abuse related and you stated that your's was new. The strangest one was a distorted piston, most of them are from spanner (wrench) slip, and man, that can really do some damage. I also suspect one of them had a distorted cylinder. Nothing would fix it.

However, if your's is new then it's probably a wire edge in the exhaust port somewhere, or someone slipped the wrench in and turned the edge a little.

You could always lap the piston to the liner with Autosol or the likes, but I'm always wary of that. Things can get somewhat pear shaped and then you have a mess to clean as well. Once again, I've done it but I wonder if it's wise to do. Is this an open exhaust cylinder or a slit exhaust ?