One of my wells uses a 4" steel casing. Just under the water line about 25 feet down there was a build up of heavy rust and scale. The rust built up due to a 5 year drought and the water level dropping some.
I found the problem pulling a pump.
My solution was to trim a chimney sweep steel brush to fit the pipe. Dropped it down with about 50# of heavy chain to pull it down and two ropes to pull upwards.
After over an hour of pulling and dropping we finally got the rust removed. The pump dropped down with minimum of effort.