The National Assembly has to ask for Competence over specific areas of legislation. It has to explain what it wishes to achieve through gaining that competence.
The UK government can say No, you can't be trusted in that area - competence will not be given!
However, having said No! Shouldn't the Union government prove that it can provide an alternative to the competence refused?
Shouldn't Westminster have better policies that address the issues raised before refusing competence to Wales?