Within the past year or so, my D3100 has begun frequently failing to shoot when the shutter button is pressed. Regardless of the mode (M/A/S) or lens, it will often do nothing. Manual or automatic focus doesn't matter (I use back-button focusing).
Sometimes, if I hold the button down for 10-20 seconds, it will may finally work.
It -seems- to happen more frequently in lower light situations (although I'm not sure), but even if that's the case, it would think that it should still take the shot, especially in Manual mode.
Any suggestions would be greatly appreciated!