Dave
It could be several things, If your eysight is OK and has been checked here are a few things to check. You say it has developed, how long have you been shooting? How old are you? What is your average?
1. Lights on the target very excessively bright? Possible but I think this is unlikely.
2. Foresight too small, a common problem. It's personal choice and dependent on how good your hold is, I think 4 mm is small, my wife shoots 90+ and uses a 4.5 mm on a short sight base, that's equivalent to 4.7 or bigger on a normal sight base. To test take out the foresight and try, I guarantee you will not have the problem then, if you do its definately a medical issue. Then start with the biggest you can get, 5 mm is usable and then reduce to preference but stay as big as you can handle.
3. Rearsight aperture too small, standard ones are around 1.1mm ish. Do you have an adjustable iris? If not could be a worthwhile investment, adjustable 1.1 to 1.6 ish is sensible. That said my wife wanted one, and then leaves it on 1.2 regardless of conditions!
4. Are you tired when you shoot? Tiredness goes to the eyes first, have you been on a computor prior to shooting, this is the kiss of death to a good sight picture.
5. Do you drink when you shoot, dehydration goes to the eyes first, and it happens very quickly. 1 litre of water per 40 shots is a good basis.
6. Re 4&5 above both are excaserbated when you are over 40 years old.
7. Is your head level-ish, the previous reply was joking but if your position has your head forced down to gain adequate cheek pressure you may be forced looking through the upper part of the eye, hopefully not your eyebrows! Try raising your cheek piece or packing if you don't have an adjustable one. Consider sight raising blocks and raise the cheek piece higher to bring the head more upright, might help your ballance as well.
Phew! I think that's enough to be working on.
Good Shootiing
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?