I don't think I can help, but
Are you using the latest version, Should say Ver 2.5B in the bottom right of the loading screen.
What level heat shield are you using? Or does it happen for all the heat shields?

I have just added the no4 heat shield to my mech, and it worked fine for me.
What other items do you have equipped to your mech?

As heat shields go, they will give you heat to absorb damage.
When your heat is critical then you can't raise your heat shield.

Take it as a safemode where mech itself protects against overheating. Since when the shield would be raised while your heat is at critical state, the next turn when opponent fires at you, your mech will overheat and you'll loose a turn due to that.

This "safemode" is in place to give you a chance not to overheat your mech during opponent's turn (only works when opponent uses physical or energy based weapons), so you can reduce your heat at your next turn.

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Aeacus