Water pumping halved after the first 30 mins in tube well-Why this decrease in pressure?(Submersible motor 10 hp; pump 14hp)

Dear team,

Good day to you. We are having an acre of land in Arasapura village of Koratagere taluk in Tumkur district. Recently we dug a tube well of 480 feet and the casing was only for 80 feet and the water was spotted at 250 feet.

Now we have purchased a submersible motor of 10 hp and pump of 14 hp. The depth of submersible motor and the pump is 400 feet.

Initial 30 to 40 minutes the water pumps with high pressure, after that the water pumping decreases to half. Kindly tell me what exactly is the problem.