Does it show as picked up in the map? Have you tried turning off you ps3 for 30 seconds or so then reloading the game?
I've ran into two glitches. Possible SPOILERS
First was in the very first camp fire, I couldn't use it but I could go further until I couldn't progress. I tried for an hour, I reloaded a check point and it took me right to the camp fire cutscene.
Second was in the white plane crash area (a village), there were two GPS caches I couldn't pick up, they showed in the map but there was no prompt. I reloaded check point, didn't work. I had to turn my ps3 off and reload the game, I could pick the caches after reloading the game.
The relic is still listed as uncheck on the map. You can use the R2 to check where the location is. But when u reach where it supposed to be. The R2 location will suddenly show u and change to another location near the evelator which there is no way u can access there :(
This happened to me and the fix was that it's in a entirely different area Go all the way back to the lower level where there's water There's a spot to climb back up and another platform to climb next to it And behind there is a relic box
Yeah if this is the relic I'm thinking of it's just hard to find. You can see it behind a rock wall with a small hole in it. Instead of going into the base, turn around and drop down into the water. Go back until you see a climbable wall with boards at the top. Use a grenade to blast the boards off then climb up. It'll lead you right to it. It's not near the elevator even though it looks like it is if you follow the marker.