It's been so long since I've Oxy-Fuel welded, I couldn't begin to offer advice, but I do have a question.
People often confuse the T-Joint "Flat" (1F) position with the "Horizontal" (2F) position.
"Horizontal" T-joints like to undercutt on the top toe, while "Flat" does not.
Are you sure that you are setting up your practice plates in the correct position?
Tim