I started to write a blog about D2 stuffiness, but when my list exceeded 15 common causes, I just gave up. Nobody wants to read 30 pages of likely causes. I'll try to keep it down to the basics. Most important for the initial examination is your make of saxophone. If it is a brand that any other person plays without a stuffy D2 problem, then it is not the horn. If you play a Yamaha, it isn't the horn. If you play a Yitzugibi, it might be the horn. The Yitzugubi might need cork crescents glued in the tone holes, or gift offerings dropped down the bell, or other strange and exhotic remedies. But if you play a brand name, then you are basically limited to the following three causes.
1. Mouthpiece incompatibility. You have a mouthpiece that causes the most unstable note (usually 2D) to choke off.
2. Mouth incompatibility. This includes both oral and lung capacity. You have some sort of weird oral cavity harmonics or undiagnosed tuberculosis.
3. Leak.
I would say #3 is about a 98% chance of being the cause. I don't care what your tech says or what your leak light shows. If other people play the same model horn without a problem, and with the same mouthpiece, and your head/mouth size is anywhere near normal for a human being, then it is a leak. Finding the leak is where my list got too long for a blog and way too long for a post.
Mark