Walkman. You are correct. No matter what, I have found my Poseidons to breathe BETTER at depth than in the shallows. I bet you feel like it just isn't delivering gas like it does at depth..almost a positive pressure-at least very low cracking pressure. I believe this is just inherent in the design of the regulator. It's even more pronounced when the tank is low-about 500 PSI it REALLY shows up. By then, I'm very shallow...ready to stand up and get out of the water.
You shouldn't have poor breathing, just not as good as at depth. I told a friend about my Poseidon with much excitement: "It even breathes better at depth than in shallow water!" and he looked at me and said "So, if you're having problems breathing, you just go deeper-right?." and he smiled.