One feels diffulcites tobreath in a deep well while cleaning it, why?

Because usually oxygen is on the ground, and then if you dig a hole, all kinds of gases scramble to run in, then oxygen is naturally less, which is also because of the depth of the ground. The deeper the ground, the more gas will press on you. Simply put, the deeper the well, the greater the pressure. Most of the carbon dioxide you emit in that well is held down by other gases and cannot go out, so that there is less oxygen.

one feels diffulcites to breath in a deep well while cleaning it because well has less amount of oxygen in it.

A fired paper piece can be thrown to check the amount of oxygen.