Trying to practice my sharp edges so I got some flexi smoothers. I’ve watched many tutorials and they seem to just glide across the fondant and sharpen up the edges. Mine just STICK, I nearly ruined a cake today because they stuck and. Pulled the fondant away from the cake.

I’ve found that if I try and do my edges right after I covered my cake with fondant, this is exactly what happens. I use a bit of cornstarch under the flexi smoothers, which really helps. But I’m not aggressive, just a gentle smoothing. I usually wait 24 hrs until my fondant is firm to really apply pressure with my flexi smoothers. 

I think the fault it the humidity, not your smoothers.  I have the same thing happen all the time with my regular fondant smoothers.  Great tip from June, waiting a day to finish the sharp edges!