Well I know he's not everyone's cup of tea but you could try reading Chek's articles on back pain
You may have to read them a few times but they've got some really good info.
For most people sore backs don't come from weakness in the lower back itself, but the lower back trying too hard to overcomphensate for other muscles. In many cases the glutes are weak and the hipflexors and hamstrings are too tight. Until you get the glutes functioning properly you can stretch all you want but the muscles will just get tight again. But this is just a for-instance, it could be a problem with the glutes, abdominals or even the lower back itself. You might want to take some more notes on the symptoms and post it in one of the injury threads.