We focus too much on breath support, when its the larynx that really needs more emphasis placed on it. Singing with a neutral larynx should be at the top of anybody's program IMO. If your larynx is neutral, it frees your voice for those ever elusive high notes and mid voice area. If your larynx is shooting up into your chin, you're choking yourself out of acoustic space and will eventually hit a point where you can't go any higher. It also causes unwanted strain, as your constrictor muscles kick in...the muscles that shoot your larynx up when you swallow. I've even found myself reverting back to old exercises from my SLS days...the "guh, guh's" and the "mum, mums"...if you've ever done SLS, you know what I'm talking about.
Anyways, I guess the point I'm trying to make is that if your having issues with your mid and head register, put your finger on your larynx and see if its moving up as you ascend in pitch. If you've made it this far into my novel, kudos to you.