Lights only sometimes working

I am creating a light in my scene using a script activated by a button press. This is working fine (creating the light) but only seems to render once. If I stop playback and then start it again and repeat the same action (without changing my script) the light no longer appears to render (although it is created).

Now if I PAUSE my game after the script has created my light and adjust any of it's attributes in the inspector the light suddenly renders in the game window and affects the plane below it as it should be doing all along.

Interestingly if I delete my plane and create a new one it starts works again (but only once, next time I run the game the light does not render)

What's going on here? Is it because I'm using the free version?

Have you checked the render settings? Number of per-pixel lights to render. I had this problem in a scene with lights turning on and off because my per-pixel lights was too low.