I've had a problem with a rear seat belt, the plastic cover over the buckle broke and slid down out of position, it makes the buckle act much the way you've described, but pulling the plastic cover up and back into its position seems to make it work correctly again. It seems something spring loaded has to press against the plastic shell that now slide out of position. No chime, because the rear seats aren't wired for the chime.
So check the plastic shell cover over the buckle. As well, some electrical cleaner to get the gunk out of the buckle may help free it up. BTW, WD-40 is insulative and cleans gunk as well, might be the best stuff to spray into that buckle and work the belt insert in and out of repeatedly to free up the mechanism.
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