Simple lamp not lit

I'm not sure if this is me of the game (v0.9.5.7) - I've made and placed two simple lamps and they aren't lit. When I did this on a different world they simple lamp was always on. I'm playing survival mode.

I've tried clicking and F-key but I'm at a loss.

Thanks for any help.

Edit: I changed computer (to my big one with real graphics) and I can see the light effect but the bulbs don't show as being lit ... maybe the light effect is my gamma setting.

It's the latter (after adjusting the brightness on one computer) - thanks for the reply!