lowpassfilter seems not to work

My lowpass filter seems to not filter, here's my code

plot(timeth,monosing,'b');
hold on
nafilter = lowpass(monosing,200,fs);
plot(timeth,nafilter,'r');
[f,P] = fourier(timeth,monosing);
[fna,Pna] = fourier(timeth,nafilter);
figure(3);
plot(f,P,'b');
hold on
plot(fna,Pna,'r');

This code should give me a lowpass filter with cutoff frequency 200 Hz but that seems to be not the case since only very high frequencies seem to have a lower amplitude in the fourier spectrum. The function fourier should work fine (already tested with other examples)

Here's the fourier spectrum graph:

[Star Strider]

By my calculations, the filter gives about a 15 dB attenuation. It would help to have your signal to experiment with.

Also, I assume ‘fourier’ is a function you wrote yourself. Note that it overshadows the Symbolic Math Toolbox function of the same name. That could be a problem if you want to use it symbolically.

[marie deseyn]

I guess that I solved it by using the function butter instead, but thanks for the effort