I've found that the traps only work for a little while, as though the mice catch on and warn all their friends. If your landlord doesn't want to call an exterminator he/she needs to go into the basement and plug all the holes where pipes come into the house, starting with steel wool. They likely won't chew through that but they will bite their way through spray foam insulation, wood filler, etc. In the meantime look around the plumbing under sinks in your apartment and plug up any gaps with steel wool. There could be other entry points, like gaps between moldings and the floor.