Do you have very thin stays? Or do you recon it's because they're just in between the notch sizes?
Just wondering as mine haven't budged.
You could maybe try cable ties just under the rubber stops.
Sorry, the mudguards don't slip on the frame, that attachment is excellent. However, if you look at the picture you'll see that the actual mudguard is held in place on the metal arms by two plastic runners - these also enable you to adjust the angle and height of the mudguards. It is these that slip on my bike. The instructions recommend drilling the mudguard and screwing the two pieces together but since I (at leas used to) swap between bikes a reasonably frequently, this wasn't an option. Tape is holding up OK for now but is just a little bit annoying.
Sorry, the mudguards don't slip on the frame, that attachment is excellent. However, if you look at the picture you'll see that the actual mudguard is held in place on the metal arms by two plastic runners - these also enable you to adjust the angle and height of the mudguards. It is these that slip on my bike. The instructions recommend drilling the mudguard and screwing the two pieces together but since I (at leas used to) swap between bikes a reasonably frequently, this wasn't an option. Tape is holding up OK for now but is just a little bit annoying.