It's probably too late, but when a pup first starts to wander, I take them out into a field with tall grass or similar and when they're not paying attention to me I hide from them. Panic sets in and they never forget this exercise. You do it as many times as it takes for the pup to learn to always keep an eye on you.
If the nick isn't working and your not getting a yelp, the collar is too loose or you need longer contact pins.
Will the dog sit steady? I walk them, sit them steady, continue walking, and then release them . On the release, catching up with me becomes the goal. This is all started earlier than 7 months creating habits that later on will be strictly enforced commands.
