At times of heavy draw on the whole marina, your voltage may be low, and the first thing to fail due to low supply voltage is always going to be your Espar heater. I had that problem with a D7L for years, until I scrapped it.
New gkiw plugs seemed to work with lower voltage than those that were a bit used, but I found that whenever I could be sure of the voltage being high, I had no problems with the heater. That was only after I had spent more on maintaining the espar than on three diesel engines, in the first 10 years I had the boat.
I also tried cleaning up older glow plugs, and that seemed to work for a while.
I have a diesel stove and I am a lot happier to have taken out the Espar altogether.