A bug introduced in v5.9 of Kodak's acquire software prevents the exposure compensation function from operating properly on v1.x and 2.x firmware DCS 520/D2000 and DCS 620 (and perhaps other camera too) files.
Specifically, the full range of exposure compensation will not be available to images shot at the highest and lowest ISOs. Photos shot with v3.09 and later firmware are not affected. Kodak is aware of the problem and working on a fix. In the meantime, revert to v5.8.2 to work around the problem if you haven't yet updated to the latest firmware in your camera.