Couple of checks to do 1st. you say you have set it a 40c check the temp on the manifolds. My stat adjuster was set a 40c and flowing at 20c, just unscrew the screw in the centre of the knob and move it on the splines so the actual temp matches the displayed temp.
I had multiple issues with flow rates and over temps etc, setting flow rates on one day was great but no good a week later. I bought those Salus auto adjustment acctuators and they solved it.
Just to note , and happy to be corrected here, my understanding is they open fully at the start and only when to flow and return reaches 10c diff do they start to slow the flow rates down to above 7c difference.