What would cause low water pressure from a deep well tank?

It sounds like you do need to put air in your tank. Look here for a fairly clear description of the process: gremcoinc.com

The exact process will depend on your system.

No. Low air in the tank would only make the pump run more. Pressure is a factor of the pump and the well. The tank will only hold the pressure built by the pump. You could have a clogged screen at the foot of your well, a bad pump, or build up in your waterlines.

*If the pressure is fluctuating, the foot valve in the bottom of the well might need replacing. This is a check type of valve that sometimes fails when sediment gets in it. They are cheap and some are made of plastic. If this is not working properly, it will allow the water to seep back into the well. This valve keeps the water pressure up between the well and the pump and/or storage tank. The tanks bladder might be faulty with a small tear in it as well.

Is there a pressure regulator in the line? I've had pilot holes plug in pressure regulators for home water systems that don't allow the spring tension of the regulator to be in sync with the water pressure it's controlling. A simple cleaning of the regulator pilot hole with a small wire solved it instantly.3

Ans 3 The respondents above have little knowledge about how a deep well system works ! First, - low air in the tank will NOT ONLY make the pump run more , -it will not give an adequate pressure to the house - regardless of how much the pump runs . Second, there is NO foot valve in a deep well system, there is a screen, but the chance of a screen becoming blocked is miniscule. The entire issue of a foot valve that will allow water to 'seep back into the well' is only relevant to a well of 70 feet or less. That is NOT a deep well !

- A 'bad pump' is simply ridiculous ! -A deep well pump will either pump or not. There are NO in-betweens !

The questioner obviously has a tank problem. -If a galvanised tank, it needs to have air inserted PROPERLY to about 30 psi. If it's a bladder tank, then the bladder is shot and tank must be replaced. There is a slim possibility that the pressure switch is faulty - or the well is not replenishing fast enough. -These issues can only be assessed by someone who knows how the system works.