Sometimes flying as Flight Leader I'll try to recall pilots by shooting flares and waving arms... actually by hitting 'R' key but I get a message that I'm not in 'AS2'. What is that? What does it mean?

ARUP, it's A2S meaning air to surface. This is the command given to your wingmen to rejoin formation when attacking ground targets. The rejoin formation works only when "ground pounding". For A2A combat wingmen will rejoin automatically once no enemy planes are in sight.

Thanks for the education! I wondered what it meant and why my guys would sometimes rejoin and other times not.