My singing voice is nicer at certain times than at other times,what is/are the cause(s) and what can i do about it?

There could be many factors, including how rested you are, how hydrated, how "warmed up" or fatigued you might be, what you're singing, etc. 

The best way to improve the consistency of your voice is to exercise your voice regularly (close to daily is best) and sing often!