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The door latch may be jammed, perhaps. It sounds rather like it.
Try entering the combo while pushing inward on the door. Rattle the door a bit, if that doesn't free it up, and try entering the combo again.
Is this the kind of (/model of) Sentry Safe that has a handle on it for opening it? if so, rattle that a bit also, in hope of dislodging whatever is jamming the safe latch.
Let me know, please. Thanks.
Then you may have to try a lock lubricant. Not grease, not oil. Silicone-based only, to avoid messing up the latch even worse. It can be found in good hardware stores.
It comes in powder form. Blow a bit of the powder into the crack in the door next to where the latch is. And, try opening the safe again.