I have a some of Bond Lee's balls which are over 5 years old. I've been noticing suddenly there seems to be a slippery coating appearing on the shells but not the balls themselves. If I rinse and dry them with a towel they're good for a few hours, then become slippery again. My pink set always did this to some extent, but my white ones just started.

Curious if anybody has had a similar experience? Is this just an effect of age? Time to buy some new shells?