Sometimes, a well-used engine will have the liner virtually "glued" in from burned fuel residues. If the liner can't be gently-pried up from the bottom, then you'll have to heat the crankcase to release it. If the liner's had damage, it may be impossible to remove it. Sometimes a damaged liner gets distorted in the case and just cannot be removed. In that case, you'd have to junk the liner and case.