Drift

Were reagents brought to room temperature prior to use?
If the reagents are not at a constant temperature prior to their addition into the wells, the results from one side of the plate to the other can differ depending on the temperatures at addition.

Was the set-up of the assay interrupted?
If the assay is interrupted at any point during the addition of reagents, it is possible that differing results will be seen before the interruption versus after. The wells that had reagents added before the interruption will have been incubating for longer than those after.