I've been coming unintentionally unclipped lately even with new cleats.
There looked to be excessive movement between the two wings on the pedals, so I thought I'd have ago at resetting they spring.
It's not very clear in the photo, but after dismantling the pedal, I found the opposite ends of the spring to be out of line.
I ground the threads of an M12 bolt down slightly so it would fit through the inside of the spring.
I then cut a notch in the bolt to take one tail of the spring and similar notch in a nut to take the other.
I then clamped the bolt in the vice with the spring engaged half way in each notch.
With one end of the spring trapped by the vice and the other end trapped by the spanner to stop them slipping out, I forced the spring anti-clockwise.
End result; It doesn't look much different in the photo, but the two tails are near enough in line now.
Only one short ride so far, but it feels like my shoes are clipping in more positively and being held more securely.
It's not like I was unclipping constantly, it only happened occasionally, so it may take a few rides to be sure that I have cured the problem.