Are the contact on the sensor fully resolved?
As in are the blips fixed, or scattered?
Targets do not show up on the contacts list on the Left MFD until they are fully resolved.
The distance this happens at is variable depending on the heat output of the ship.
A Diamondback scout running cold will not resolve until you are very close, vs a Type 7 charging to jump will show up further than the listed range of your sensors.