Adjusting a completed pledge

We have a pledge on which all payments have been made. I need to adjust a portion of the total amount to a different campaign and designation. When I go to do the adjustment and I enter the original source, the box pops up that prompts me to select which gift to adjust, however the one I need to adjust does not appear. I'm not sure why it wouldn't show up or how I can get it to show up so that I can adjust it. Any ideas here on why this might be happening?