Creating a programmable gain amplifier with a digital pot and an op amp
What are you driving it with? With the inverting configuration the input impedance equals Rin, so the input impedance will only be 200&Omega;. This means unless your source is pretty low impedance (e.g. &lt;20&Omega;) it will be noticeably attenuated.
Also, make sure you are using the digital pot correctly (I haven't checked how it works) by swapping it with a fixed 200&Omega; resistor - if it works okay like this then it must be something to do with the pot setup.