Hi Chris @TheSkwee ,
The thing with Clamp Overlap in the Bevel Modifier, is, that it stops all Beveling, as soon as, somewhere in the Mesh, two Vertices are so close together, that they would ‘overlap’, when Beveled:

It’s a bit like a safeguard, that is being over-protective.
I almost always turn Clamp Overlap off and use something like Weights to Limit, where Beveling should occur.
So-called ‘double Vertices’ are a classical example of when Clamp Overlap creates no visible Bevel at all.
Not Applying Scale can also cause problems.