I've looked at plenty of books and websites , looked at scales, triads, etc, and I can competently play a few bass lines when I have the tab. However, there is a missing link- I have trouble relating the tab back to the theory. i.e If I look at the tab to a bass line, although I may be able to play it, I can't instantly see what scale or triad is being used. Another way to put it :"How do you know where the use of one scale starts and another ends?"
Do I make any sense?