I find this strange, that shellac will seal the timber but lacquer wont.
I was taught that "Sanding Sealer" really just takes the place of coat number 1, but in the case of gloss finishes, it will cut back easier. I have never used it and neither have any of the polishers I have worked with.
The only explanation that I can think of regarding the lacquer soaking in and requiring so many coats, is that it was thinned to much.
Glad you found a solution to the problem.