If you're not in an area where other aircraft are being interrogated by either ATC radar or TCAS units, then you won't see them on PCAS.
Not much of a problem this side of the Pond.
I've also had hundreds of false warnings, "ghost" images, etc.
You were probably using one of the earlier units. I have run mine alongside TAS and over around 10 hours I did have a single false warning.