From what I can tell it looks like the hot water comes in from the top line and heads into the manifold but also down to the mixing/regulating valve on the bottom line. After the water runs through the system it comes out to the return line and back out to the heating system.
My first thought is to try closing the blue valve on the return a bit and give it a minute. My thought process is that reducing the cold/out flow will help the circulating pump pull the water from the mixing valve. You could also try messing with the regulator to see if the temperature changes when you adjust it.
I’m not necessarily familiar with subfloor heating systems but I’ll give it a shot. I found this video that helped sort some of it out.
https://youtu.be/OSJkjcTHVLE?si=aGyvO6ue3Ng0FT9I
From what I can tell it looks like the hot water comes in from the top line and heads into the manifold but also down to the mixing/regulating valve on the bottom line. After the water runs through the system it comes out to the return line and back out to the heating system.
My first thought is to try closing the blue valve on the return a bit and give it a minute. My thought process is that reducing the cold/out flow will help the circulating pump pull the water from the mixing valve. You could also try messing with the regulator to see if the temperature changes when you adjust it.