Holding glow plugs on longer than a slow 10 count is a bit unusual... it shouldn't take nearly a minute to get things warmed enough to start.
Beyond that it's difficult to speculate - first thought is that your glow plug switch itself may be intermittent.. try to check if you're truly drawing current with the glow plugs switched on (often you'll hear a change in tone of the buzzer as the plugs load down your DC system...)
One other comment - if this is a regular occurrence I'd get into the habit of keeping your raw water through hull closed until the engine actually catches, otherwise you run the risk of back flooding the engine through the exhaust after repeated unsuccessful attempts to start.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"