I have been welding SA335 P91  pipes of OD 558  and thickness around 80 to 90 mm.Root pass is by Tig process followed by  two  layers of SMAW welding.I also  purge the joint with argon  to prevent root oxidation  while welding .Subsequently by SubArc Process to complete the welding .After PWHT  radiography is taken  which does not show any indication .How ever the UT exam shows indications   mostly 3 to 10 mm from the root .Before UT grinding is done on the inside of the pipe to smoothen out  any excess penetration . What could the the problem and the solution ?