There could be lots of reasons why VM will not install in certain flats. Some of them might be administrative, some technical.
We see regular topics on here where VM will not install at heights above first floor level when the cable runs externally. If the VM cable needs to run internally we have seen past issues where either VM is not interested (due to cost) or the freeholder/managing company does not grant permission.
There have also been past topics where some flat owners have managed to get a connection because they live on the ground/first floor, and could get a cable run externally, but others above first floor could not get a connection. Hence you end up with a building where some flats are omitted but others are serviceable.
There have also been past cases where the old cable TV companies originally installed cables across roofs to provide a service but VM now refuses roof access for safety reasons. This has also meant some existing customers could not get repairs to cables. The list of possibilities goes on and on.
There is no particular reason why the sales people should know the reason that a particular address is not serviceable. All they know is it does not appear on their list.
If you think that you can establish that a VM connection might be possible (and not prevented by things like the examples above) then VM does sometimes send spotters to check out locations but that seems to be a fairly hit and miss process as to whether anything happens or not and the customer in waiting has really had to push hard to engage VM to make it happen.