Problem soldering stained glass?

I just made my first stained glass window and assembled it on a piece of 3/4″ exterior plywood with wooden stops along two sides to hold it while I put it all together. I was able to solder the leading without any problem but when I turned it over and attempted to solder the back side (the side that had been resting on the plywood for about three weeks) the solder would not take to the leading, even though I cleaned it and applied flux the same way I did on the first side. The solder beaded up and rolled off the lead and would not adhere. I am wondering if vapors from the plywood (such as fomaldehyde) might have somehow chemically altered the lead so as to prevent 60/40 solder from bonding properly. Has anyone experienced anything like this? And why when I clean it the solder will still not stick? I have been soldering things for 30 years and I have NEVER had this problem before.