Clean 'em again. I read somewhere that something happens with the copper if it is allowed to set for a period of time before soldering. I never even bother to clean the fittings and pipe ends or apply flux until I actually am ready to solder the joint. If I can I'll do both ends of the fitting at once, but if that's not practical, I treat them individually. I normally use a round wire brush that fits inside the fitting and another doughnut-like brush that fits around outside of the pipe. Emery cloth works very well on pipe ends, but is harder to use inside fittings. I use a small brush for applying flux on the pipe ends as well as inside the fitting.