Hm that's a bit odd, it wouldn't pick up VGA for me I had to use a DVI input on mine to work, I can't remember where i saw it but I'm sure i read that you should use the same inputs to minimise problems.
Which one is the middle monitor? I connect my middle one using active mDP to DVI and the other 2 are DVI to DVI and passive mDP to DVI, it wouldn't work using a DVI to VGA wire/adapter for me at least!
Left: DVI to VGA
Middle: DVI to DVI
Right: active mDP to DVI