Oh yes, the old FWB 150 I just got had the front globe full of dust, cleaned it out. Most of my guns have to stand upright on the buts so I put a cotton plug in the front of the diopter and then a small plastic zip lock over the muzzel end covering the globe sight.@crowski
Try an adjustable iris eyepiece the first chance that you get to try one!
It sounds like the aperture is too small for your eyes in one of your eye pieces. You can gauge them both and see if there is a difference, and open up the smaller of the two with an appropriately sized drill bit.
DO NOT forget to blow out the inside of the rear sight where the eyepiece screws into, then are notorious for collecting dust, stray hair, dead bugs etc.
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