I also agree with Flags, bugling activity depends on general conditions, weather, herd dynamics, predators, hunting pressure, and a whole host of other things. You can have a ton of elk in an area but they are not bugling for whatever reason. It can also light up once a couple cows come in to heat or diminish quickly as the rut tapers down.
As for the OP the remedy for your situation is to maximize the opportunities you are getting. When a bull bugles that is really just the first pitch of the first inning, close the distance and really work the bull and commit to him for the day trying your best to get an evening shot opportunity. Never leave a bugling bull to go try to find another one unless you absolutely have no play on that bull.