[sdiy] Solder stops flowing all of a sudden?

[Karl Ekdahl]

Hi list, so i've stumbled on this peculiar thing where my solder stops flowing all of a sudden. It melts, but doesn't become liquid, it sort of becomes "old" immediately the way solder becomes if its been on your tip for too long - giving horrible crack-ily joints. It seems my iron is keeping its heat and the tip is brand new, i seem to recall stumbling upon this before and it was a roll of solder that was bum halfway through.. rings any bells? Not sure what to do, buy another roll of solder? Is my iron going? 

Thanks

Karl