Keep in mind also that if an engine begins to overheat, it will go lean as well. So if it is properly tuned, but in a bad cowl situation, you will start to lean out as the flight goes on. Adding nitro, adds heat as well, so that could have been a factor. If you do not have a cowl, then it sounds like the increase in nitro caused you to go lean. Is the increase in nitro the only change? By that I mean is it exactly the same fuel otherwise? Same oil percentage, same kind of oil, all synthetic, some castor etc. This matters a lot. Keep in mind that the fuel flowing through your glow engine is as much of the coolant as anything. If you increase nitro, and there is a reduction in oil, then the temps could have increased. Then you could go lean from the heat.
I think the simple answer though, is to just make sure you get the needle set right, and the pinch is the best way to do that. You should note a slight increase in RPM, If it sags, and then comes back, you are lean and you need to open a couple of clicks, then try again.
Interesting, I have a brand new one of these .65 AX engines that I have not run yet. I will be flying it for the first time within a week hopefully. It is mounted on a new Bridi, Sun Fli 5 model. I am going to try 11-7, 11-8, and 12-6 props. I can report back which I like the best. I have also added a Macs one piece black muffler, as I hate those fragile 2 piece things that come with them stock. Not sure how that effects power, but I am going to guess I may lose a little.