The fact that you welded something earlier that didn't give you any problems is informative but not conclusive. The time difference introduces the possibility of something else having changed. If you would get some other steel that you are confident is okay---the closer the dimensions to the steel you're having problems with, the better---and put it right beside the problem steel, you can very quickly isolate the problem to the material or to the welder.
NCWayne said he and several other people spent 30 minutes trying to troubleshoot a welder problem that turned out to be the wrong gas. Having the chance to rule out a whole range of possible problems is an opportunity you shouldn't pass up.