In my experience, this is usually caused by the repeater doing double duty as a wireline keyed base station. In other words, it only has a squelch tail when a mobile or portable unit is transmitting to the repeater. When dispatch keys the radio, they are doing it over some type of wireline or wireless telephony circuit. This means that the transmitter does not have a received RF signal to manage a squelch circuit on. Think of the repeater as two radios, one TX and one rx. If there's no rx signal, then no squelch tail.