Are you receiving (not tracking) the system in conventional mode?
If you don't hear a control channel, then you won't be able to track the system. On the other hand, if you are receiving the simulcast signals from more than one site, then your scanner will act as if it was being jammed. A directional antenna will serve two purposes: 1) bringing the signal level high enough to track and monitor and 2) emphasizing one site over the others, hopefully enough so that co-channel signals will not interfere.