As someone said they stretch over time. So when you first put them on, and adjust them to the right tension, they shoudl be all good, then as they over the next couple of hundred k's, you redadjust them again.
How does grease help the problem? I can see how grease would stop the noise (by lubricating), but I can't see the grease making the belt "grip" the pulley better. Unless it gets absorbed into the rubber over time and moistens it up or something?????? Kind of like, when you drive with wet shoes, it's slippery at first, but after it starts drying out it gets to a REALLY grippy stage....