mahlkonig grinder (gbs) keeps taking longer and longer to grind

I've been working at this coffee shop for 3 years, and the thing is this one grinder on the left keeps increasing the time required to grind the same amount. fuuny thing is both of them- 10s and 5.6s, gives same amount of coffee, 18-19g. I'm just a part timer so i don't know much about detailed settings. my boss clean them regularly and changed the blade recently. Does anyone have clue what is wrong?