Either the leather is stretching prematurely which is causing the bolt assembly to be lose, or the bolt assembly is loose in which case you can tighten it and probably be ok. If it's the leather you'll find more and more of the tension bolt will be engaged as the leather stretches, in which case your saddle will probably have a short life. Sometimes this just happens. Leather has some variation, and it's possible the leather on your particular saddle wasn't very durable. I'd mark the threads with a marker or something. If you notice it rattling soon after you tighten it and you have to engage more of that bolt I'd send it back. I'm 200 pounds and have about 1500 miles on my Brooks. I've never touched the bolt and it's doesn't look much different than the day I bought it.