600 mts shots

[Fournoses]

good evening fellow hunters,

after i made 2 very long shots, 500+ meters, and several 400+ shots, im struggling with the next challenge: 600+ meters... the problem is i cannot spawn animals (sitka deer, dall sheep) at more than 550 mts... i cannot understand if my pc isn't enough powerful as far as graphic card is corcerned, or if i m using a wrong tactic.... i've spend many hours without any spawning after 550 meters....
any suggestion will be appreciated,
thank you

have a good time and good hunt,
fournoses