Ty,
I have not had this exact problem but... Nioro cannot help?

I have solved similar but not identical difficulties of my own by changing my cutting and finishing tools which were apparently contaminated; I presume your first step was to insure cleanliness of the joint area but you must be sure of your tools. I consider it a kind of virgin sacrifice.

Also, Welding Journal had a generally similar problem they advised on about a year ago- less than two years- which I think had something to do with temperature and/or the amount (too much) of paste. I checked through what few recent back issues I can find with no luck, however... that would put it more than 6 months ago I think.

Good luck and I am interested in how you solve the trouble.
D