What is Simplex

(S)=Simplex, same frequency for tx and rx and no (+) on your display meaning no offset.

If you can not hear the other station using that same simplex station you are using that only means you are out of range of that other station because a simplex station is not repeating what you say locally. The RF is traveling from that other station to you and not being repeated through a repeater. But you will hear the stations calling in from the IRLP, All-Star because that is coming out of the simplex radio to which you are tuned.