It sounds like your bladder is shot if in fact you are losing air over time. Make sure however that your air pressure gauge and your water pressure gauge are both reading the same. If you are getting water trapped on the wrong side of the bladder, it will reduce your drawdown.
My bladderless 30 gallon tank is a precipitation tank for iron removal and it provides about 5 gallons of drawdown.
If your checkvalve in the pump failed, you would be losing water back to the well which would be evidenced as a dropping of pressure when there is no water usage in the house. If the pressure holds, that is not the case.