Well, it depends a bit on what shapeshifting is like for your Druid. It's worth chatting with them about what it feels like, how it relates to the spirits, what they have to do to manage things and so forth. But off the top of my head - say the spirits withhold that animal or that domain until some future point. Or have them shift into a different animal. Or have them shift and get stuck. Or have them partially shift and be stuck with stubby, useless wings. Or have some of their belongings shift into them. Or attract the attention of malign spirits.
It's been a while since I DM'd. Failed Healing moves were always my least favourite, I'm glad you're rocking those.