The basic problem with multiplayer.... any kind of lag or high ping rate will kill missile lock. As will "gazelling" (i.e. enemies zipping around suddenly).
I never use missiles for this reason (my ping 450-500), I might as well fling credits at the enemy. Although of course people who have very small ping rate have much better success. I found torps far more effective, when the bogie is coming head on at me.
EDIT: Just to expound, I have 80-90% hit rate with torps this way. Even with cheap, tiny Starkiller (the weakest torp) I can take out L.18 VHFs with 2-3 hits at most, and lesser ones with a single shot. Or you can use something heavier... but be careful of splash damage! And remember to punch the bogie's shields down first.
