quiet A string?

[wellREDman]

One of my students has come in with a bass he has aquired that has a problem that has me scratching my head. The A string is really quiet compared to the other strings, If it was on just one pickup I would put it down to a dodgy pup, but the problem manifests with either pickup on full with the other killed (although its far worse on just the bridge pickup) I find it hard to believe that both would develop the same fault, does anyone have any Ideas?

[Aussie Mark]

If a different set of strings doesn't fix the problem, it might be that the "A" nut slot is a little tight, and needs slight filing to widen it a tad.

[Dave W]

Or it could be a burr on the bridge saddle. You've ruled out the pickup. If it's not a dead string, then something at the nut or bridge is killing the vibration.

[wagdog]

If it's a Fender style make sure it's wound correctly on the post to ensure a good break angle.

[wellREDman]

cheers guys,
when I looked closely at the nut under the string  there is a chip of plastic missing, a bit of a file has improved things a bit, so I'm confident a new nut  will do the trick