Sam- the Russian planes have a safety built in that prevents the release of a weapon until the firing (or release) envelope meets MINIMUM reqt's. That is- given a continuance of this aspect- a hit should occur from a certain point forward. Once you cross that line- you'll get "LA" or "nP" on your HUD.
Since I saw you online in Hyperlobby last night- a couple of tips/ reminders. (I don't know exactly how new you are) Remember that the ONLY so called "fire & forget" air to air missile in the "Russian" FLYABLE inventory is the R-77, which is carried by the MiG-29S. ALL other radar guided missiles for flyable Russian planes are SEMI-active- which means you MUST guide it all the way to impact or miss. Heaters (73, 27t, & et) are always active- but for closer range and prefer rear aspect shots.
Some ground attack weapons use laser- riding (your laser) some are fire & forget. The ideal firing envelopes, ranges, capabilites, and tactics vary WIDELY. This knowledge will come to you with practice and as these guys have already said- reading & studying the manual.
ONLINE: If you use a pylon editor- you might want to forget you have that. Do not get caught using weapons that are "illegal". No amount of noob-ness will save you from the wrath.