Lilly growth question

Last year I put a bunch of lillies in shallow pans (round, oil pans) with
rocks.

Our other lillies are in containers with a minimum of a foot of rock depth.
They have been thriving. Large leaves and flowers.

A few of the 'pan' lillies came out of their pans (think koi visits) and
fell to the bottom of the pond.

The pan lillies have small leaves. They are doing OK. The ones that fell
onto the CEMENT bottom of the pond are thriving...big leaves. Their roots
have some muck on them, but are mostly just on the bottom cement.

Any thoughts about why they do better bareroot on cement than in rocks/

Jim