First Order Filter - Voltage Follower Attenuation
Very simple answer. Your op amp is meant to receive signals 0V to 5V but AC signals can go to negative, example +1V to -1V.

What you need is capacitor in series and bias to 2.5V, so your signals would be between 1.5V to 3.5V. Which allows the op amp to work properly.

You may download this pdf for reference: http://www.analog.com/en/analog-dialogue/articles/avoiding-op-amp-instability-problems.html