you have to place it on grass
and have grass for the mooshrooms to spawn on

 

We have done that but it has yet to even spawn 1 single shroomcow.
We tried on top of grass and in grass with no luck at all :-(
Thank you for trying though.

 

flat land 9x9 grass directly under the spawner I can't remember if it needs to be open to the sky or covered but if you cover at least 3 open above it.

 

oh have you tried mycilium under it?

 

Ahhh... that may just be the ticket if @freezing34 has not tried this yet!

 

I think I sell in the mall at /warp bird if you need.