I had this problem with an exposed bolt on a door that opened outwards, once the door was open, I examined the bolt and noticed that there was a small cut on the angled face of the bolt which caught the blade on the knife and made it feel like an anti jimmy (it wasn't that good a lock - ERA rim mounted body with a Yale cylinder fitted - I really love carpenters and DIYers

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Opened it with a slim jim used as a shove knife. Try working the bolt from underneath with the two knife trick or a THIN piece of steel cut into a "J" shape with the hook of the "J" fairly sharp.
If you can see the bolt - look to see if there is a small peg which sticks out either above or underneath the bolt or if the bolt seems to be made of three pieces of metal. If so that's the anti jimmy.
If you have an anti jimmy, it probably has harded steel (anti drill) pins inside so that idea is out unless you have hardplate bits. You will have to use either picks, letterbox kit or a locksmith the other option is that you are left with brute force and ignorance (ie a carpenter)