Have you removed the scale from inside the pipe. This may be very tightly adhering and cannot be removed with a flapper wheel. A grinder should be used.
If the pipe is clean, verify the torch shielding gas is ARGON and that no leaks exist. If the torch isa flex head, verify that the rubber does not have any cracks that may open up if the torch head is moved. If that is OK then verify the flow rate is not so high that air is drawn into the weld. Smaller Gas cups require less gas flow.
Some 70S-x compositions may not be suitable for the steel you are welding on. The suffix -2 -3 -6 etc all have slightly different purposes. This may be a contributing factor.
Verify that no internal draft exists in the pipe. This can often cause shielding gas be removed too quickly.
Thats all I can think of.
Have a nice day
Gerald