You need to get all 4 pressure plates to be active at the same time. You can walk on them systematically to determine the correct sequence, or...
Numbering the chests from closest to farthest as you enter that dead end, the order is 3, 1, 4, 2.
If that is what you did but the barrier still didn't drop, try leaving the cave and returning to solve the plate puzzle again, and see if that helps.
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