Slump or drape?
You say that athe pieces were “slumped over”. Does that mean that the glass was to drape over the mould on the outside? Normally a slump would be to the inside of the mould.
If it is a drape, then a ceramic mould is often difficult to get out, especially if there is no or a negative draft to the mould.
The other thing I have found is that some moulds are being used for slumping that are intended for casting, and so are more difficult to use.