I can't emphasize the importance of face slapping and restraints in training good little cock suckers to properly encase their masters flesh in their mouths and into their throats.
A proper little cock sucker will grunt approvingly when master rubs the back of their throat with the head of his tool.....
Protests of a gag reflex should be met with shift and sharp face slaps to shock the cock sucker into focusing on the task at hand.
If this doesn't work.. a good caning and ass fucking should help.
In extreme cases a head down ass up fist fucking should do the job of setting the cock sucker to task.