I am a bit confused here as if you have dropped a trade the trainers should teach you a new trade and the only thing i can think of is this
Or possibly you have the "Show" settings in the trainer's window set to something that isn't showing the apprentice skill? Make sure it shows "Already trained," "Available," AND "Unavailable."
as all the low area trainers should teach you and i assume you did check those settings ?? , i know i have changed trades on a few characters and had the settings not show me i could train when in reality i could after changing the settings.