This is something that is happening to often to me with Atc on training servers.
Approach giving me directions and tower asking me to contact them. Then i contact them and they not answer. When i land it lock like there is no air planes. I see it on the map but not on that airport beside a white square.
I place a video on this link if any body have an answer about this
Im a grade 3 and i use expert server only whe is real busy. But most of the time training server have more traffic so i flight on it some time.