Man I wish I had xray vision. Ill do my best to paint a picture. this is a Family pool that i have been around it since it was built, Uncle was care taker for the last 5 or 6 years, now I am. There are 5 eyeball returns. 3(deep end)have good pressure and feel somewhat equal to me, 2(shallow end) have no flow at all or at least undetectable by my hands. Pool was built in the mid to late 80's so I was just a kid when it went in. I do remember swimming in the pool as recent as the late 90's early 2000's and there being pressure on all the returns. My uncle had installed a fitting in one eyeball last year to install a fountain. I have replaced it with a normal eye. i also removed all the eyes to check for blockage at the eye. All is clear. Only changes in the life of the pool is very recently i replaced the pump, 2hp Hayward and the sand was replaced last summer by my uncle. however I dont think there was pressure at the eyes prior to the new pump. There is a always 15-20 PSI in the filter, both new and old pump(old was also 2hp). Ill double check the pressure when i get there.
From what my Dad can remember from the build the return line runs around the pool and T's at each eyeball, which eye is number 1 is a mystery but I assume the one with the most pressure to be it. My thought is somehow maybe at the #4 there is a blockage and not allowing it or #5 to have flow. I dont think it is broken because I would think we would be having a ton of water loss and that isnt the case.
From the filter pad there are 2 return lines. The normal eyeballs and a fountain. Fountain works great and does not send flow to the eyes not working when it is opened so ruled out them being tied in with the fountain
my only thought is to try and fish something through the eyes and feel for a block or blow air from the eyes back to the pad and hope to blow something out or loosen it up with reverse pressure. For years they always lowered the water below the returns but never plugged them off. Im thinking maybe something nested in lines and somehow blocked it up.
I realize this was long winded and probably impossible to diagnose on the internet but im just looking for some ideas to try.