I know using the prong seems basic, but you might want to consider a trainer for proper walking/heeling and using the prong. Did you placed the prong on correctly? High on the neck, below the ears, not tool loose and the prongs should not be placed under the neck, that where the rings should go so you don't damage trachea.
A quick "pop" is all you need. Are you encouraging your dog to heel or just letting it go wherever on a flexi leash? Although all dogs are different, I've used these on both my large dogs starting as puppies and they never showed any discomfort or made squealing noises. They do offer excellent control when used correctly.