Do you have edited spell.dbc in your custom patch?Do you have edited spell.dbc in your server data folder?Did you restart server after all changes?Did you delete Cache?Does your spell work when you try to learn and use it?
Would you provide soke kind of screenshot what your row in npc_trainer looks like?
Are you sure that your NPC has all kind of flags set correctly?
I think there is a spell flag that prevents it being used in a trainer. Trainers are purely server side too I believe. I'm not entirely sure.
Trinity is using the skillline*.dbcs to generate trainer skills iirc