It would help if you could provide more info about your data: how many samples, sample rate, and a plot of the (unfiltered) data. A spectral plot would also be helpful. Is your bandpass filter FIR or IIR? What are the filter coefficients? Also, a plot of the filter's magnitude and group delay response would be helpful.
What happened to the 8 kHz region?
First we don't know the properties of your signal. Your parameters may or may not capture relevant information. The frequency range of interest is important, you have to be sure that your bandpass filter range includes the signal.
Finally your pattern recognition/machine learning algorithm has a role, how you trained it and what algorithm you use are important.
From the little information about the signal you supply I would simply guess that your assumption about the region of interest is wrong - obviously the low-pass throughs away relevant discrimination information, so maybe the component at 8kHz does significantly contribute information.