It is also in the very nature of the welding process. Using an electric arc emanating from a conical point, aimed at an inside corner joint, the walls of the joint are gonna be closer to the tungsten than the tip is to the root of the joint (this is especially true if you hold too long of an arc length-----hence why Jody is almost always saying in his videos "you need to hold a tight tight arc!). Electricity will therefore have a tendency to initially take the shortest path to ground, and there you have it: arc wander onto the walls and not into the root of the joint.
Some things can help with that:
- bump up the current by at least 30-50%
- reduce the cleaning action (not always possible because then you get contamination, but if you can, it will help a tad)
- increase the AC Frequency
- hold a tight arc!
Yes it is true sometimes you do simply need to add just a dab of filler to help bridge the gap, then get that puddle flowing into the root so you get good penetration.