The trouble with completely passive filters is their high output impedance. Or in plain layman's terms, their output is "weak". The filter's performance varies dramatically (both gain and frequency response) depending on whatever load is connected to its output. The impedance (or strength) of the input can also matter.
My guess is you probably weren't aware of this, otherwise you would have included more info in your question about what's really connected to both sides of the filter. We can assume the input connects directly to an Arduino pin, which is a fairly known quantity, though the output from older boards is somewhat different than the newer models.
If the load connected to your filter output is reliably known, perhaps the filter can be designed to take it into account. But only so much is possible without adding an amplifier into the filter. To drive low impedance (or "heavy") loads like heaphones or speakers, or to get consistent performance with different loads, an amplifier is needed.