My control cells are dying. Why?

[Elioelio]

I am growing cells (DLD1 and HCT116 colon cancer cells) on 96 well plates.
To eliminate edge effects I'm putting sterilised water in the outer wells.
I have 5 wells with just media, then treat the rest of the cells with the drug I'm testing. I use a Wst cell proliferation assay (a colorimetric assay) and measure in a plate reader.

However I'm having problems..
1. The cells in the wells F2-3 and G2-3 always appear dead (even when just media is added). As in the cells appear as dots, and are very sparse in the centre of the plate.

2. In the last assay, all my control cells appear dead too (in wells B2-6).

I have been doing these assays for a year now, and have only been having problems in the last few months.

Any ideas????