TEPCO still hasn't identified the cause for the decreased water flow into the reactors (1, 2, and 3), but the company suspects some rust or foreign particles clogging the system somewhere. On September 5, a video camera was submerged into one of the buffer tanks that store the treated water before the water is fed back into the reactors.
White particles floating around, like near the bottom of the ocean. I wonder what they are. TEPCO explains the bubbling seen in the video as "nitrogen".
Here's a photo of the strainer of one of the 5 "chiller" machines (to cool water), from TEPCO's 9/5/2012 Photos and Videos page. Rust flakes and curled-up white bits. (It looks like bits of jellyfish...) Click to enlarge.