I've never had to swap my rabbits' hay as they've all been on grass hay mixtures from the start, but here are some pointers that i've read others mention.
How long ago did you first put timothy in with the alfalfa? Some rabbits are stubborn and will wait days or even weeks to taste a new thing.
You can try offering singular strands of tim. to see if she'll take it, maybe even gently poke her with it until she gets frustrated and tries to take it. Another option can be slowly swapping the ratios of alfalfa to grass hay. Add some tim hay every time you refresh hay and reduce the amount of alfalfa by that. Then, when her alfalfa runs out and she's hungry, there's nothing to eat except the new grass hay. With that, you do need to be on the lookout that she wouldn't go too long without eating any hay.