Can't use simultaneous audio inputs anymore

Hello folks --

I USED to be able to talk on Skype using my mic input, record my line input with Audacity, record my Stereo Mix with another program, and record my USB audio interface with yet another recording program -- all at the same time. This was all using the built-in cheapo Realtek High Definition sound on my motherboard. I do streaming and broadcasting and need to do all this and was very happy with the setup.

SUDDENLY, I can't do that anymore. Windows now allows me only to record off two things at once maximum, sometimes three. I used to be able to do four at once if you include Stereo Mix and my USB audio interface. Now I can only use my mic and the stereo mix -- or the stereo mix and the USB interface -- or the mic and the USB interface -- but generally ONLY two at a time. The ones that work have to be set to "default device" or "default communications device" for them to work. The only time I can get THREE to work simultaneously is if I set my USB interface to "default device" and the mic to "default communications device." That allows me to record off Stereo Mix too for some reason. But the main line input becomes dead and no recording program can record from it.

It seems as if my system has two A/D converters, so I can only convert two analogue stereo audio signals at a time. I understand that. But my system USED to ALSO understand that neither the USB interface nor the Stereo Mix tie up an ADC, so they can be used simultaneously with the Line and Mic inputs. But now it stopped understanding that!

Why would things suddenly change like this? What can be done about it, if anything?