Denon AVR 5803

If there is "large/small speaker" settings on your receiver, maybe you set speakers on Large. Some receivers, when set speakers to "large", won't redirect signal on subwoofer.

I have similar problems with my sub connection on my 2805. My sub activates now and then when very low frequencies are being played, but only when the volume is about -35db or louder. The surround EQ might have been changed on your receiver. I have a feeling that the 2805 has a weak connection to the sub out jack so another possibility is that you have a shorted-out wire. Couple of suggestions: check to make sure "cinema EQ" is not on, as it compresses the frequencies to the upper and lower dynamic ranges, which might mean LFEs are not being played. Also check your crossover frequency and set it to somewhere higher (200Hz is a relatively high crossover frequency that would be good with very small main speakers, while 80 hz is a low crossover point that would just add a little extra oomph to gigantic speakers capable of playing very low bass).