Joined: Jul 2014
Posts: 1,397
Likes: 169
From: Milton Keynes, England
Bikes: 2.6kw GT LTS e-tandem, 250w Voodoo, 250w solar recumbent trike, 3-speed shopper, Merlin ol/skl mtb, 80cc Ellswick
Are the threads good? It's only a 5mm thread, if it's bad on either the male or female thread then it may be jumping a thread during riding.
The lever itself should be closed all the way to it's stop (note any disc brake or spoke issues and use the drive side).
And ensure it takes a firm push, though i always found my thumb was just strong enough.