Johnson Lifts — Malfunction of lift even after servicing

Address:

Chitturi Metro Apartment, Tuni

We have paid AMC for our two lifts in apartment. Service technician from visakhapatnam Johnson's &Johnson visits every 45 days fir service. He says no problem with lift. But the first lift stops very often. Upon complaining he says that there is a power supply defect, but the second lift runs smoothly when the first lift stops.
What shall we do ?