Sounds to me as though your hide building skills are a little lacking, so it will definitely pay to look at what you do and how you do it.
Anyone who tries to shoot from a cramped position or a hide that only allows limited movement will end up suffering in the same way as you? and that includes accuracy.
Clearly there will be a limit to the size of the hide you can ? or should ? build without alerting the birds to your presence, but a comfortable set up enables you to sit in a relaxed manner with sufficient elbow room to mount the gun smoothly, and reload it without forever getting the muzzles tangled in netting.
No two decoying situations are ever the same of course, but our aim should always be to create a hide that allows us to sit easily, in an upright position, with the top of the net level with our forehead.
We can now watch incoming birds calmly through the netting and then rise easily to mount the gun and fire it in a slow, smooth, movement that isn?t noticed by the quarry.
One sure fire way of straining the back and legs is to use a seat that?s too low and puts our knees above the level of our backside, or sitting with our feet on ground that?s sloping upward and raising our knees in similar fashion.
To stop this happening you should dig a small hole to take the feet thereby allowing us to sit properly.