I have noticed that spotting planes below, especially 109s and 190s seems often quite hard, maybe it is an rendering issue. My wingman and I often fly quite some time over an objective and eventually spot one plane and wait a bit after if there are more, but once we attack that one plane then all the other planes appear around us the moment we dive in and infact we get into a quite crowded area which was not seen at 4000m above the objective before. They simply seem to render too late or never.
On the other hand spotting russian or british planes which use a camo as well is quite easy, they seem to shine, i don't know why.
Since I fly in real life and i am often confronted with military helicopters nearby flying low over our dark forrests with their dark camouflage. Therefore and at least for me it is easy to spot them just looking into the square they are flying, maybe because the human eye is REALLY good at seeing moving objectives, which infact are the helicopters moving way faster in relation to everything around. The distance is like 4 or 5 km, but i am not so good at telling the distance to them.
I can imagine that spotting is one of the hardest things to model in a flight sim.