What makes you think you're shooting high?? Just a feeling??
Since you've been hunting 20 years, let's assume you didn't just sprout to 6'-4" in the last year, & your gun fits the same as every other year.
If you're right, I think the most likely culprit is that for some reason, your face isn't meeting your stock.
Obviously you have to lower your head some, but ideally your stock should come up to meet your face; not lowering your face to meet the stock. If you're having to lower your face to the stock, maybe there's a reason the stock isn't getting up high enough. Getting caught on loose clothing?? Maybe you recently added a thick butt pad to compensate for longer arms??
Another couple reasons I think we tend to not get the stock up are:
1. Tall cover. If we're in tall cover, it's easy to want to stretch out, thinking we have to in order to see the bird above the weeds. Head & stock don't meet.
2. Gimmes. We're so eager to see those easy ones go up in a puff of feathers that we lift our heads slightly to enjoy the view that never happens.
3. And the biggest reason.....rushing things. The ability to relax when a rooster flushes comes with experience/repetition, but sometimes you have to consciously force yourself to breathe & take an extra second to really see the bird BEFORE mounting the gun completely. I think the better your brain sees the bird & its actual flight path, the more likely your mount will be a good one.
A friend told me about these problems. Luckily I've never experienced any of them myself.