I had one where the check valve failed, and blocked the discharge.
I also am believer in dropping the discharge point below the run so
it will drain the line and help eliminate need for a check valve.

On Nov 13, 9:22*pm, "EXT" wrote:
I bet you have a check valve. When the pump shuts down the silt/clay filled
water just sits there and the silt/clay settles out and sticks to the pipe.
Try removing the check valve and add a tee just where the pipe exits at its
highest point with a vertical piece of pipe and a drain air valve (or a pump
foot valve) so that the water drains from the pipe and sucks air in through
the valve. This way the silt/clay cannot settle out inside the pipe.