Homage inverter hexa 1004 problem

When theres loadshedding my inverter start dropping load gradually and shuts down in 2-3 minutes and does not turn on again. I have taken it to the repair a couple of times and he does something with the ic with soldering tool. Its down again and to make it start again i need to take it to repair shop. Any ideas why the ups does it and it does not even turn on when i have main power supply on after its ups is shut down by load shedding.