OK, what make is it? S&G? LaGard? Other?
Make sure you are dialing the sequence correctly. You need to make sure the dial is cleared by rotating at LEAST 4 complete turns before stopping at the first number; then depending on who the maker is you will need to pass the next number twice before stopping on it, and then pass the next number once before stopping on it.
Also, the less expensive dial combinations can lose their index by a number or two. Try a combination that is off the "correct" combination by 1 digit each way, each number. That works a surprising amount of the time.
It is possible that the relocker has been engaged, at which point you'll need to have it drilled.
It's none of those manufacturers. .. sentry has a direct entry proprietary lock of poor quality. With zero knowledge of the combination, I can manipulate this open without drilling any holes in under 45 minutes. Assuming it is a properly operating container with no mechanical issues. I really wouldn't keep anything of value in these safes.