Starlight doesn't work properly in first person. In order to get it to work, you either have to look straight down while in first person, or switch to third person. However, it should work in both first and third person immediately upon casting the spell.
Also, it displays other odd behaviour. For example, disappearing after one second. Upon further research, some claim that this specific issue with Starlight (disappearing after one second) is due to wearing anything that is enchanted.