Are the pots different sizes? Is the one that's not doing well generally a bit bigger than the others? Is the failing one in a different situation in terms of wind, sun/shade?

No the pots are the same size they were bought about 2 months apart. And they are both situated in the same location

Reminds me of two Skimma's I bought at the same time, same pot, same position etc. One started throwing out new shoots and looked great, the other didn't do anything. Last week I tipped out the sad one and there in the soil was one of the little white grubs who had happily munched through most of the roots, no wonder the poor thing was hanging on only! Just a thought, might be an invader.

As Honeysuckle's cautionary tale suggests - check the roots, assuming you can't find anything that shouldn't be there on the bark, branches or leaves.