Op-amp circuit has lower gain at low frequencies
If you ignore the input capacitor and look at the input impedance into your op-amp circuit from the left of the 200k resistor, the input impedance is the 200k resistor. This is because the op-amp is configured as a virtual earth amplifier.

In other words there is 200k loading your capacitor and the 3db high pass cut off point is when Xc = 200k ohms (in magnitude). This equates to: -

Frequency = \$\dfrac{1}{2\pi RC}\$ = 79.6 Hz

In other words below this frequency the signal level coming out of your amp falls at 6 dB per octave hence, at about 40 Hz the output signal will be about 6 dB down and at 20Hz the output signal will be 12 dB down.


  At 20 Hz, the gain is basically 1.


Midband gain is about 5 (1 Mohm / 200kohm) and this, in decibel terms is about 14 dB hence, at 20 Hz there is a net gain of about 2 dB (1.26:1 in real numbers). Make C bigger in value.