Recently purchased a used kemppi. Used it for 20 mins all good. Came back to it the next day only just melts the wire nowhere near enough power to weld anything whatever power setting it is on.
Normally on a transformer-based MIG with low power issues, I would start by looking for loose, burnt, or corroded joints and connections in the low-voltage high-current secondary circuit - i.e. the transformer secondary, the rectifier diodes, the choke ( inductor ), and the torch connector and work clamp & return cable. When high current is being drawn, any excess resistance caused by a faulty joint will cause voltage to be dropped across it.
However, I think the Kempomat has fully-variable power output, controlled by a potentiometer rather than the typical discrete steps of a rotary switch. There may be an open-circuit diode, or a fault in the thyristor ( SCR ) output control circuit. If checking for faulty joints does not solve the problem, then measure the DC output voltage from the torch tip to the work clamp as you try to increase the power setting from low to high.