The goal of endorsement policy to set the rules about the number of peers / orgs which must reach on an agreement on the execution of a chaincode, given a set of inputs. Signatures are used as a means to ensure that the response was not tampered with and used to identity which org/peer actually responded.
You will not have the case where a peer will not "sign" the endorsement response, unless you have a malicious peer where someone has actually written there own version of the peer code. Things which can occur are:
If an organization has Admin Role in a channel, then while it signs the transaction received from the clients of that channel, it would use the Admin Certificate to sign the proposal.
As per my understanding, the Roles" Admin & Member" considered in the Endorsement policy are as the ones mentioned against the Organization section "Role.Admin" and "Role.Member" in the channel configuration. And not the roles of the individual users of the Organization.