She's can't change because she's already a Perfect Cherry Blossom

poor lily

it's a spell that makes all the bigness go to koakuma only

The laws of cuteness prohibits Lily from becoming beeg. She is forever stuck being smol. I'd say there are much worse fates out there than that!

Magic Circle: Sorry, we already have a Daiyousei.

But seriously... it could be out of juice. Or maybe it only works on devils and youkai. Looking back, I also notice that all three have entered at different angles: Koakuma at 6 o'clock, Kyoko around 3 o'clock, and Lily around 11 o'clock.

Yet another tiny fury, same as Koa.

But, for a brief moment, she had hope.

Seems like smolification won't work on something already smol!