did you change the shutter button from focus to release?
I saw this mentioned on one tutorial I watched, however it was for a Canon and I could not find this on my camera...Nikon D3100. I am not really sure what it means or how to change it. Another tutorial I watched specifically for my camera body said to switch my camera to AF-ON which is what I did.
The shutter button is still set to only fire if the focus has been acheived. If you use set the AF-L button to AF ON, but fail to change the function of teh shutter button itself, it won't allow you to activate the shutter if when camera doesn't think the image is in focus (green dot).
seems like the D3100 can't disable the option. Are you shooting in AF-C? Try AF-S. But this defeats the real benefit of BBF.
Or trying holding the shutter button down part way, BBF, recompose, then hit the shutter all the way.
This is just a simple firmware limitation that Nikon decided to exclude from the cheaper cameras for no reason. Nikon likes to spend manhours removing code from firmware because why innovate when you can rest on laurels.