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I see that circulator in your photo. This diagram is how your system might me piped:

You can see there is not a lot of pressure drop across the mixing valve when the MAIN pump is operating because is pulling equally on the cold inlet of the mixing valve and the return from the radiant floor loops. There is nothing to move the water into the cold side of the mixing valve, so all you get is what might pass thru the mix valve top the radiant loop and the rest of the flow may even go in the opposite direction on the mixing valve cold inlet port.

This next illustration indicates the circulator in the proper location.

The pump will pull water from both the hot and the cold ports to get the proper water temperature and move it thru the radiant floor tubes. I have also learned that you do not need the circulator pump where you actually have it located. That is because the mixing valve will pull the water from the boiler as needed to maintain the "MIX" temperature you set it for. If it needs more or less hot water that valve will compensate automatically.