Originally Posted by Raiyn
What you're describing happens when a square taper crank is ridden for some time with a loose crank bolt. (Probably from missing it's first tuneup or even from being missed on assembly. There's no guarantee when it happened)
It's not noticeable at first but slowly the Bottom Bracket (BB) spindle (being made of hardened steel) begins to hog out the hole in the softer (Aluminum) crank. By the time you notice it it's too late.
Tightening the crank arm pushes it a small bit up the taper of the BB spindle which gives you only a small area (Like a thread or two's width0 of "new" contact on a higher (fatter) part of the spindle. The fact that the rest of the hole has been hogged out means that with in a short time your crank will start flopping again. Tightening it again will have the same reaction. Usually after about the third time tightening it you'll wail on the bolt causing it to shear. (Always fun)
Bottom line: Your crank on that side is hosed Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself
Informative post Raiyn. Thanks.