So you tried ... if at first you
don't succeed ... try, try again!
Maybe try a different method ...
I highly doubt that a chicken will just go and gobble up a pile of clam shells ... no way to fit it down ... TRY smashing them up into 1/4" chunks with a hammer on concrete ... wear safety glasses/goggles ...
On your soft/mushy food experience ... did they also have other things to choose from to eat, or did they ONLY have mush? For how long?
TRY this ... a little wetted feed with treats on top, no other food available ... by day two they will learn.
Then TRY to add some cooked scrambled eggs to the wetted feed ... yummie they will say!