Boiler mystery - recommended technician?

Hi, Our upstairs tenant’s boiler is an enerjet gas furnace which was working perfectly for many years until the end of last winter. The furnace seems on and operable; we replaced the upstairs thermostat and checked electrical work. All seems fine – but the thermostat won’t activate the heat for some inexplicable reason. We’ve had an electrician and two plumbers come by to try to fix – they all leave saying its fixed, but the next day, it doesn’t work. Any recommendation on a trusted person to come by and give it a look? Thanks!