The clothing feature is probably more complex and buggy than it really needs to be. If you assign a new image to a slave while their image state is either pregnant or naked, then their image will not change until their image state returns to normal. A slave will have a naked image type if they have been stripped of at least 2/3 of their clothes. If the image is changing, but reverting then they are probably choosing to not be naked.
The intent of a slave to chose to be clothed is primarily determined by their exhibitionism fetish opinion. If their opinion of being naked is high, then they will remain naked. If their opinion of being naked is moderate, then they can be forced to remain naked using the nudity rule associated with the advanced brand (acquired by progressing the story quests or always available in sandbox mode). If their opinion of being naked is low, then they may choose to re-cloth even though you attempted to force them to be naked.