A. Make the stand in so large. This makes him harder to cover.
B. Have contant frame movement. It's hard enough blending things standing still. Adding motion makes thing more complicated.
C. Have an actor contently move in front of the effects area. They had trouble with Han's finger using CGI, it would be harder with film compositing. not only would the character have to fit, they would have to cut out Han in each frame.
This leaves two conclusions.
1. The stand-in was Jabba. It wasn't an effects shot. Lucas cut it for story flow. In 97, he needed what Walt Disney called "a wienie"for his re-release so he put Jabba back in. Except, when he did RotJ, he decided to make Jabba a slug. Making a non-effects shot a effects shot.
2. Lucas is truly a horrible director at special effects. Wanting a complex stop motion puppet, then filming the scene in a way that makes inserting said puppet impossible. Even I can't imagine he's that short-sighted.
Of course, there's a third possibilty. That Lucas changed his mind during filming, and decided he wanted a puppet instead of a human. Only Lucas knows. One thing's for sure, it's not an effects shot. In the end, it doesn't change the fact that it's the single, worst part of the film.
Please, offer your own opinion. As long as it's your opinion, and not, "George said......"