lower mph at track??

The last couple of times I've been to the track I haven't been able to get the same trap speed on motor, as I did from say...last year. My mph has dropped by about 2 or 3mph. What could this be from? Should I mess with my timing? It's on 15 degrees btdc right now. I've adjusted it some but didn't really feel or see a difference bumping it up or down by a few degrees. My rebuilt motor has about 20K miles on it...and has only had 2 & a half bottles of nitrous run through it over the past few years. My nitrous system is set up correctly and working properly so I'd like to believe that the juice isn't causing excessive wear in my motor. Plugs look fine. Check engine light isn't even on. I've been told that I'm running a bit too much fuel psi. being that I have 24lb. injectors and still run close to the stock fuel psi. Yes my MAM is calibrated for the 24s. Currently my fuel psi. is set on 38lbs. w/ vacuum line attached. I'm going to take it down to around 32-34psi. and see if that does anything. Could running my car at the track in different weather cause a loss in mph? It was much colder with a lot of moisture in the air the last couple times I went to the track.