Were these sets already in service or newly installed? It is possible that the digital sets are either connected to Analog circuits or that the digital circuits are not correctly programmed.
I have also had two cases where the wiring was a problem. One the cable had an open, the other was one jack looped to another and both had digital sets (you can only have one digital set per digital circuit on the Mitel).
Because you have screen that is active, that means that the sets are actually receiving power from the Mitel. Check that the extensions are programmed and that the wiring is good.
If that doesn't work, I would suggest taking one of the sets and rigging up a jack to come right off the equipment cabling (ie. The Bix Strip or R66 Block). This will eliminate a potential wiring problem. If the set comes up, you have a wiring problem. If the set still does not come up, you have either a programming problem or you have not used the proper wire pair off the equipment.