Bluestone Discoloration

I put down a bluestone patio and had no unusual discoloration, some light reddish brown, blue gray, gray, etc. Normal to me. About 2 weeks after the job was completed a couple of the stones seem to be browning and leeching a brownish substance to the joints. Pressure washing prove to take the discoloration off the joints, which said to me it was not migrating from below. The stone seemed to clean a bit. Another 2, or so weeks past and the same situation has occured. I'm stumped. Any help is appreciated. Thanx, Mike.
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