What I think he was saying is that as the sight rises, the angle from the front sight to the apature's hole becomes more and more of an oblique angle...reiar sight rises straignt up, so you have to look thorugh the tiny hole at an angle to see the front sight.
Is possible, with that particular sight, that you are getting interfearnece from the deep recess to the hole. Apature has a long screw shank that goes into the staff...it's pretty small diameter...and it's possible that the angle from the tiny hole to the fornt sight is partly blocked by the end of that long recess. It's like looking through a long tube at an angle...eventually you'll get to an angle that just lets you see the inside of the tube (tube being the long shank of the apature disc). Making the recess shorter (but still giving full thread engagment) might clear the path.
Would guess that the rear sight may not be mounted quite right (or the detent is placed so that it does raise the staff at a true 90degree angle to the barrel)....if mounted with a bit of backwards slant, you'd get the abvoe blockage problem earlier in the staff's elevation. Could check for ture 90degrees with a ture right angle when detented.
If I remeber that sight right, there is a flat spring with a crease in it in the base of that sight...the crease acting as the detent. Can try dissassembly and flipping that flat spring around...which will change the angle of detent. MAy make it worse, may make it better, but it doesn't require any modifications to try it.
Ahve ground that flat spring's end, making it loose in it's seat, and found the true 90 degree stance for some odd ball rifles...once found, have to make a filler (welding prefered) in the spring's recess so it's not loose, butsets that flat spring to detent at the right spot.
Another way is to shim the mount until you get that 90degree stance..this is easier, can be fillded with with differnt thicknesses, and can be made pretty unobtrusive if you try.
OR
Filing the base to a new angle...which sounds easy, but getitng it countoured to match the seating spot is a lot harder than it seems.