Hi all! I'm a tenant in a wonderful old house, and we got a new tankless water heater about a year ago. There's a bit of a problem with it that I've complained about several times but the plumbers seem to be unable to fix. Here's the issue:

Neither the kitchen tap or the bathroom sink tap get hot water when they're alone. So if I want to wash my face with hot water, I need to turn on the shower. If I want to wash dishes, I also need to turn the dishwasher on, to get hot water. I can run the tap for 15 mins and just get cold water, and if I listen to the heater, it's not turned on.

The plumbers have tried to figure it out, but of course, when they're here, usually to fix something else, all sorts of faucets have been run, so there is hot water..so they think I'm crazy.

What I think is that there's not enough water pressure to turn the heater on when it's just the taps. They both get low water pressure.

The water heater is a Takagi T-K Jr.

Should I contact the manufacturer to see if there's something I can do on the device so it would turn on more easily?

Or should I just continue running two taps and waste water?
Please help, my landlady and plumbers think I'm making this up, but I'm not!