I don't know what's the reason why I can't use the stingray homing missile on my revenant ship. I have the licence for it now. for some reason if i press the "c" for secondary weapon it fires the primary instead. Anybody knows what's the problem why I can't use the missile? Is it a bug or some sort? I know I remember using it one time and it works and now it's not working at all.

Check your weapon groups -- that's what determines which weapon is fired by which trigger. You get to the weapon groups page via docking with a station and going to something like Ships->Configuration->Groups (that is, the Groups button is on the same page where you can drag addons onto your ship).

Also, note that if you're using the very nifty Ship Loadouts Manager plugin, it does have a bug that makes it not auto-set your secondary trigger when buying an unsaved loadout.