This could be due to low pressure in the storage tank. If the air bladder looses pressure, the water pressure will also be reduced. When empty the tank pressure should be 7 psi. If it won't maintain this pressure, the tank will need to be replaced. Another possibility could be low water pressure to the system. The incoming water pressure should be a minimum of 40 psi. A specially calibrated low pressure air gauge will be needed and also a water pressure gauge. A technician may be needed to make these checks. I'm listing a site you can check to have a technician out. Here is a link for the website:
Sears Home Services
. For specific information about your system, please resubmit your question and a complete model number will be needed.