Some cats really need a "hidey-hole" to be comfortable and at ease (especially in new settings). You might create a fake one or two for the cats to discover and let them hide there as much as they desire. There are some
soft-sided cat beds that you could try. It might be enough to make her feel protected and secure without your having to fight to get her out of it. The one I linked to is really soft (it may fall over if not on a sturdy surface), but can be squished into an open bed (and washed).
One of the big things with adult cats is that if they hide from fear/nervousness/etc., you don't want to force them out into the open - it can do more damage to the cat's psyche than good.