Hayward H400 heater won't relight.

I had a new Hayward Heater (H400) installed today. The heater initially fire up fine. Reached 100 degrees on the spa, cooled to 93 I when I checked the heater the "IF" (ignition failure) was blinking. I tried firing it again with the same result. An hour later, after it recirculated the pool water into the system which was cold, the heater fired up. I didn't let it get to 100 degrees this time but rather called the installer. He came back out to look at it and told me that it runs for a bit and then looses gas supply.
He is going to call Hayward on Monday. In the mean time, I am trying to troubleshoot myself.

My previous heater which died after ten years had a similar problem towards the end if its life. When we moved into the house two years ago everything worked fine. Several months ago I noticed the heater would get the spa to temp but would not kick back on when the temp dropped.

Do I need to have a pressure drop test done? Problem with the gas line or the heater?

Any thoughts?

Hi and welcome to the forum.
My guess would be a failing pressure regulator. Since the heater was working that says the pipe size is fine so an undersize feed pipe would not be the problem. You don't say if it's natural gas or propane. If it's nat gas then if you have other gas appliances check to see if the gas pressure is dropping on one of those when the heater tries to fire. I'm saying this because of what the guy told you. 'Loses gas supply'. There would be a main regulator somewhere for the house and they do fail. Infrequently but they do. If propane then there is a regulator at the tank. That could be failing. There is probably another stage of regulation in the heater itself and this is the one I would suspect. You could also have somehow gotten crud in the pipe which could be messing up the regulator. If you look at the plumbing, just before the pipe goes into the heater there is usually a TEE with a verticle pipe going down with a cap on it. That's a sediment trap to hopefully catch any heavy particulates that make it that far, before they go into the heater.
In a nutshell, I don't think you need a new heater if all else like the burners, heat exchanger, and other parts look OK.
Let's see where we go from here.
Hope this helps.
Al