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This is a current flow issue. Typically what we find is on the front turn signal sockets the little tabs down inside the socket are compressed too far to apply sufficient pressure on to the bottom of the bulb and the little ground tab on the side of the socket isn't strong enough over time and this causes low current flow the bulb will still flash but not as bright. If you take the socket out and wiggle the bulb with the turn signals on you can make them flash brighter and slower. Typically it's just the front ones that are affected.
I normally just use a small screwdriver to bend the tabs up and out on the side to make good contact. Just make sure the powers not on to the socket or you will blow the fuse.