Hi there

I have regularly used the large syringes of solder paste without any problems. However, I recently bought new torches and small syringes of solder paste for the class I teach - hard, med and easy - and it doesn't seem to work well at all, quite often the clay part just burns black but doesn't burn off and the solder doesn't melt. We've had numerous problems with it in class which has been frustrating, especially for nervous students. I've tested the same solder paste myself and sometimes it works and sometimes it doesn't, and seems to work best when the area is fluxed with Auflux before and on top of the paste. But the whole point surely is not to have to do that! I am wondering of the mix has changed, or if the torches have, or something else... However, Cooksons say not! Any ideas as to what the problem could be? Thank you.