To Blackhawk’s comment’s first:
Older Dixon’s had a different headjoint design that didn’t work so well, so Mr. Dixon Scrapped that and came up with something else, which works pretty well. I have no idea which model Darvis tried, but it could have been one of the older ones.
Another problem could have been a headjoint mismatched to the body, there’s been some discussion on that here in the past, and that’s something the retialer would have been responsible for.
Another possibility, if you truly couldn’t get a note Darvis, is that there wasn’t enough grease on the slide, and it was leaking.
Bill,
In all seriousness, is this your first Low D? The Dixon is pretty quiet, does have very low air requirements and needs very little air presssure, however the one I had responded fine. I mean it’s no Overton or Copeland, that’s for sure, but it’s an inexpensive Low whistle, and performs very well with that in mind.
Based on your description, I’d say there’s probably nothing wrong with the Dixon, it’s just not your style. Depending on your Budget, Try The Copeland, Overton, or if you have thick fingers and less money, the Howard Low D’s.
The Overton will give you plenty of warning before the octave jumps, and has the most volume across it’s range - Copelands tend to be a little quiet on the bottom end by comparison Copeland Low D’s are generally considered most “Flute Like” in tone, and are perhaps the most elegant looking whistles on the planet. They also produce excellent volume, but be ready to work your lungs on the fast tunes 
The Howard is perhaps the best sounding inexpensive Low D on the market, and as I recall, the tuning was quite good. It just has holes that are a bit too large and far apart for many beginners. If you have thick fingers, go for it!
At any rate, there’s no shame in returning the Dixon if it’s not what you’re looking for, I’d just say don’t expect a replacement to play much different.
Loren