When I had my pipe out a few weeks back I didn't realize there was a rubber bumper that sits under the pipe until I got it back in. Maybe check for that? Mine fell off on removal and was sitting in the bottom of the nosecone.
Some sleds need a washer or two between the standoff on the pipe and the rubber foot that sits on the bulkhead. Most quality aftermarket pipes come with a few washers for that purpose or at least used to.