Which effects zirconia shades more?

Hello Bridgesters!
Here's another question for you... our full contour zirconia crowns have recently started looking less translucent than before and a little on the yellow side. We were debating the cause and have come up with two main ideas - either we're putting too many units in the sintering dish at night, or it's time to change the beads in the dish. What do you guys think? Throw out the beads or cut down the load, or something else we haven't thought? Thanks!