UFH zones not heating up

[Jim Norf]

Hi, I'm a complete novice to UFH so excuse my ignorance! I've just had underfloor heating installed and an area of the ground floor is not heating up. The thermostat is set to 21 but it isn't getting past 20, and that's due to heat from other zones within a large open plan ground floor. The pins on the manifold seem to be stuck up on these zones compared to the others. The installer has had a look and says that the thermostat needs to be moved as the heat from the other zones is affecting it, but to my understanding surely if the thermostat is set to 21 and its only reading a room temperature of 20 the floor should be heating up?! Picture of the manifolds and the pins for the zones attached, the far left 2 are the areas that are not heating up. Any knowledge that anyone can impart would be much appreciated