Yep there fun to feed
Are you feeding live or frozen thawed prey.
It is a bit risky but giveing your burm live might be all she needs to awaken the preditor in her.
If your feeding frozen thawed you can try super heating your rat in hot water then rubbing a fresh killed adult mouse all over the rat the sent might fool her.
Or after giveing her the largest mouse you have , just as its going down slip a medium rat in right behind it, Its called chain feeding but it likely wont change her not wanting to take rats right away it could take a few times for her to get to know that they are food to .I had a friend who,s burm would take nothing but baby bunnys , We switched her up by chain feeding her like that .
There all different so you never know whatll work, im sure some of the others here can give you more idea's to try as well.
Hope this will help you out.