Sounds like a process control matter to me.
Nothing is more weldable than 1018 and the Drawing Quality 14 gauge shouldn't present any problems.
Things to look for:
Inconsistant surface conditions in the weld zone.
Lot to Lot variations in oxide/scale are common enough and might produce the results your trying to fix. Adjustments (if Logics) can be programmed into your sub-routines to deal with these vairiations, or more simply you can provide surface prep that will produce consistancy in the weld zone regardless of the condition of your steel as recieved.
ER70S-6 deals better with scale/oxides than ER70-2 or 3