| This might be hard to explain without diagrams, but the bridge pins of an acoustic guitar (bass or otherwise) are not designed to hold the string in by pressure alone. There is a reason there is a slot in those pins — it is to allow the string to pass through, but not the ball end.
The ball end is supposed to be right up against the underside of the bridge. The bridge pin is really there to hold the string and ball end up against the side of the hole in the bridge. (Theoretically, you should be able to pull out the bridge pins once the strings are tuned up to pitch, the ball end should be caught under the bridge. NOTE: I do *NOT* recommend you try it. It *will* work itself loose, and you'll put yer eye out!)
So the pin is there to apply lateral pressure to the string/ball end, not vertical.
That being said, tapewounds like La Bella and Rotosounds are rather fat. They may not fit all that well against your typical bridge pin. Make that slot in the pin a little bigger? Reach through the sound hole and make sure the ball end is up against the side of the pin and not caught on the bottom of the pin?
Make sense? | 
