calibrate theromstat? what?

we have a bristan pisa thermostatic mixer shower replaced cartridge 5mnths ago shower is cold again have checked it seems to be fine so took it back. we were asked if the cartridge was recalibrated before it was put back in, what do they mean? shower worked fine then just stopped the next day its driving us mad moved into house 18mnths ago had nothing but trouble with the shower since.