Bass signal falls off

We all use kempers, two guitarist and bass player. We noticed the bass player was hitting a note and it would ring and shut off like a noise gate closing. He tried turning off all effects and dialing off the nous gate. It still seems to be there any ideas what could be causing this? Imagine hitting an open A string it rings for a few seconds then dies like a gate kills it??? I'm puzzled nothing in the signal path and all effects off???? Thanks in advance.