They have a screen at the cold water inlet. Sometimes that gets clogged up some.
See if that needs cleaned I have had to clean mine a couple times it will reduce your flow.

They also have restrictor washer at the clod water inlet. If you know you have warmer water you can increase the size of this washer this will let more flow.

Yeah I figured there might be a water restrictor.
I always check all new equipment and fixtures for water restrictors and remove them.
I didn't see any on the water heater.
Since I did install clean outs on the inlet and outlets of the water heater I can turn off the water and open the clean outs and send my probe camera in there to see.
It has been a year or 2 since that hot water line has been used much, so maybe the inlet screen is clogging with rust.

On very windy days the hot water heater likes to fault out and not light the burner. Or it will light for about a second and go out then fault.
Yes I have a chimney cap.

I don’t recall what brand and model you have.

The one I have is a marley.
I think it's the Lowes brand.

I'm using this tank less to fill our hot tub.
By my my calculations it's putting out 73,000btu per hour of hot water. That's like a commercial hot water heater.
I thought it would shut off after 20 or 30 minutes, nope it just keeps going and going and going.

If you need a lot of hot water this is the way to go.

In the next house we are getting I don't think I will use a tank less water heater on account of they waste too much water.

double post

How do they waste water ? never thought about the coming into play.

I was wanting to install one in this house until factored in having to upgrade the power supply so swapped in a new tank and called it done.

For the next place thinking a solar water heater for summer and a pellet / wood stove water heater for the wet winter weather. They could share a heavily insulated storage tank

The tankless waste water by the nature of their opperation.
When the water starts flowing it has to flow for so many seconds before the burner lights, then when it lights it has to get up to temperature.
Unlike a tank water heater where the water is at temperature already, so the hot water starts flowing as soon as the first drop.