You'll have to be more specific by what you mean "it doesn't recognize it".
Are you talking about when you are using Audyssey? Or do you mean that in normal use it's not playing?
Also, what is the make and model of the sub and speakers?
Not to worry, we'll hopefully get to the bottom of it.
Just some general things here:
(note: I have a alittle bit older Denon, the menus should more or less be the same, but maybe not exact, so if you don't see what I'm talking about look around a bit. If you are still having trouble I'll download the manual so I can give specific instructions)
So, first: Make sure the gain on your subwoofer is set to half way, and if there's a crossover setting, turn it as high as it can go (or if there is a defeat switch turn it on).
After that, on the 5010: Go into setup, select "speakers" option then "manual setup" then "speaker config". Once there, make sure that the front/center/surrounds are set to "small" and "sub" is set to "yes".
After that, go back up one menu so you back into "Speakers/manual setup" and go into crossovers. Here you'll need to know how low your speakers play, if they play lower than 80hz, select 80Hz for the crossovers. If you are using smaller satellites that don't play that low, set the crossover to what they are rated at...or the next higher setting (for instance, if the speaker goes to 110Hz, but the crossover only lets you choose 100 or 120, pick the 120).
Next, go back up a level again, this time choose "bass". Once there, for "subwoofer mode" make sure "LFE" is selected and the "LPF for LFE" is set at the same level as your "crossovers"
Last thing: Go back up to the main setup menu, from there select "Audio" then "subwoofer level". Set it to +0dB.
It should play at this point. If it's not, make sure your connections are correct: Should be "sub out" to "line in"(LFE), if it is, maybe it's a bad cable, try a different one (trying using both R and L line in also). You can use just plain RCA cable too for testing (if you happen to be using a coax cable currently).
If it's still not playing, then somethings not working right. If you have another receiver about try that, you can us the tape out if there's no dedicated "sub out" just turn the gain all the way down, play something and slowly turn the gain up. You can use a mp3 player too this way if you have a 3.5mm jack to stereo RCA adapter. With this, you can keep the gain at half and turn the mp3 down all the way, then put something on and slowly turn it up.
If it works that way, then the 5010 is most likely defective, otherwise if it still doesn't work, then it's most likely the sub.
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The above is for trouble shooting, if you can get the sub to work with the 5010, you'll still need to make final adjustments and/or run Audyssey (which you really should do anyways).