Sallen-Key Low Pass filter
First of all in your circuit you are using 1pF capacitors? this is too little for any practical circuit, even the parasitic capacitance of a PCB is higher. Second, with 1pF and 10Kohm resistors your cutoff frequency is 15.92MHz, the opamp you are using has a GBP of 0.7MHz, can you see a problem with this? with the specified gain the -3dB point of the opamp is around 270KHz, and if the opamp uses a single pole compensation the roll off will be 20dB/Dec, so what you are seeing is the opamp rolling off the gain long before it reaches the 15.92MHz cut-off frequency. With a 1nF cap the cut-off frequency is 15.92KHz, well within the bandwidth of the opamp.