imac brightness control is stuck at half

The brightness control on my brand new iMac 27" appears to be "stuck" at half-way. The display is not very bright and I'd like to increase it, but when I press the F2 brightness key, it will go down, but not up past half-way. Is this a retina display thing or indicative of some problem. It does appear dark and I'd like to brighten it up some.