Had several, all in one day, on a pronghorn hunt with my newly built 25-06 on a 1903 action. First with my handloads, and then with two brands of factory loads I borrowed. The delays were maybe 2-4 seconds. It was a frustrating day because I saw the biggest buck I've ever seen, and, hangfire. He took off and I couldn't keep him covered long enough.
Later, I took the rifle to my gunsmith friend. He discovered a tiny ridge of wood that swelled with the heat, and was delaying the cocking knob. A few minutes with sandpaper and some linseed oil solved the problem. This was over 20 years ago.