I picked out a new mask from my LDS that I've dealt with for years, tried it on in the store and it seemed to fit. My first opportunity, (the next week), I dove with it in the pool, with a reg. in my mouth and it leaked around the nose. After all adjustments i determined it was not a good fit so I returned to the same shop to exchange it for one that might fit better and was given a hard time. Am I asking to much from my LDS??
No, as you describe the situation, you are not.
Obviously, there may be another 'side to the story', but given the events and timeline, unless the mask was somehow damaged when you returned it, it would seem reasonable for the shop to make an
exchange - which is what you requested. We certainly would in our shop.
I agree with speaking directly with the owner. Don't threaten the owner with the possibility of losing your business, etc. That approach usually just tends to piss people off, and seldom produces a positive, mutually beneficial resolution. Rather, give the owner a chance to correct the situation. You shouldn't have to remind the owner of the amount of business you have done or brought in - it is the owner's job to remember.
If that doesn't produce satisfactory results, thank the owner for their time, and express disappointment that a reasonable resolution was not achieved, leave and don't go back.
If it does produce satisfactory results, thank the owner enthusiastically and continue to do business there. You will be viewed as a good customer.