Take a look at the Tenax fastener. In the hole, you should see three ( I think is is three ) metal leaves. If one of these has broken off and is partially obstructing the hole, the thing won't grip. Its usually caused by people part pulling out the Tenax knob as they push to over the static element. The partially retracted leaves then get distorted and ultimately break.
I push the Tenax over the static piece without pulling on the knob - they last a lot longer that way.
Mrs F is no longer allowed near them.
They are easy to replace - I got a bag of 10 and the tool you need off eBay.
