Low Pass Filtered Noise Power Spectra, Showing Low Power @ Lowest Frequencies
There are a few things that could be happening to be honest.

1) Your Op Amp is a real Op Amp. As such, it has limits for what type of loads it can drive (They aren't all unity stable, they aren't all capable of driving capacitive or inductive loads, etc.)

2) Your ADC input impedance is messing up your "power delivered." In Electrical circuits, loads have an effect on the filter frequencies. ADC's also aren't really known for having resistive inputs.

Personally, I think it's the second one, but it doesn't really matter 'cause you solve both the same way. Move the filter to before the Voltage buffer (follower) and see if that fixes the problem. It should look like this:







simulate this circuit &ndash; Schematic created using CircuitLab