Hii, im pretty new here on Vatsim, i noticed when im in airborne i don’t see any aircraft around me(when vat-spy said i have a couple near me, when i landed i can see all the aircraft around me(i use FSTL Traffic files) its any solution for that?
I cant speak for everybody, but when in the air i can visually see aircraft only when aircraft are about 10nm or less from me.
Add to this, on my EHSI my TCAS shows the aircraft a lot further out.
How close are these aircraft to you? This could simply be a LoD setting that unloads elements if they are further away than some predefined distance - and aircraft in the air will usually be quite a bit further away from you than on the ground.
Though I can tell you that MSFS pales in comparison to the real world when trying to see other aircraft, even at the highest possible LoD settings 
Like 2-3 miles, i was landing and i was another aircraft on parallel and i didn’t see him until i was rolling out