I have a set up similar to your's, except had a 2HP Polaris Blower --and also had issues for the first 3 years of installation. Assuming this was an issue you had from the beginning of the pool, it would sound to me like it's an installation problem.
Here's my two questions, and why I ask them:
1) How far is the pump to the spa? If its very far, then the blower may be insufficient. (mine is 50 ft-- with only a 2 or 3 pipe bends..and the 2HP provides plenty of power -- I would bet tho, that is not the problem.
2) Is your Hartford Loop above the water line? The Hartford loop (an upside-down plumbing "trap") is necessary to restrict the water to travel back the air pipe when the air/water mix is met with back pressure-- like when u sit in front of the jets. This is necessary so the blower isn't overworked pushing water that's in the airline. This is not the job of the check valve. The Check Valve is just to prevent water from going into the blower, at the blower. If the blower is well above the water line, I doubt a check valve is even necessary. This was my problem -- the Hartford loop was in the spa wall, a great idea, but, it was not above the water line-- a mistake by the pool builder-- and it lead to water always in the air line, created constant load on the blower, the spa never worked well, as it had to push water all the time, and killed the blower after a few years. So, check that out.
3) Is the Venturi "T" installed in the right direction? The Venturi T is the plumbing part that mixes the air from the blower, and the water from the jet booster pump. It's plumbed as the pipes enter the spa-- or, like mine: the air and water is mixed at the jets. If your design has a Venturi T, check that out.
4) Blockage: What could it be? A dead animal? A rock or something that fell in during contruction? I doubt it would be that. I imagine you may see what that might be if u use a snake or a blower/string setup that plumbers use to pull wire through pipe.
Disclaimer Note: I am not a pool pro, but I am an engineer and understand the basics. The above things is what I learned about my setup after hours of research-- your situation may be different. I had to find my problem myself, as the pool builder was becoming non responsive -- it did force me to learn a few things. He did fix it finally, when I pointed the problem out. What a difference when it works correctly!
Good luck.