You have to drag bench players to designated slots (not reserves). Subs work only for injured players when you are off-line. If you are watching the game, subs are turned off. Fwd are subs for strikers only, Mid are for mid-fielders (MC, DMC AMC, ML & MR), Wng are slots for wingers (AML, AMR, DML & DMR), Def for all defenders in the back line and Gk for goalies...