My Leave Keeps Getting Denied!

I work for a rapidly growing research company and I am the only RN on nights. About two months ago I requested leave a month in advance, for two days off. One week before my leave was due I get a letter from my nurse manager saying she couldnt grant my leave. Reason behind it was because no other RN was willing to take up my shift for those days. Okay so I missed out on precious time with my fiance, who is a marine stationed far away from me. He was only in town for a week, and I only requested two days off. That was strike one....
Strike two! I put in two days of leave about two weeks ago (kind of short notice, I know) for a spur of the moment family event for this upcoming weekend May 17-19. Surprise! Last night I got a note from my manager stating my leave was denied! My family is extremely upset, seeing as I will not be at a remembrance for my older brother who passed away.
Seeing as I have a wedding coming up, a honeymoon, and all that goes with that, I am quite worried about getting leave for these things. Day shift and evening shift RNs have NO problem getting their leave approved.

My questions to you all are:
Do any other night shift nurses have problems with getting the leave they request?
What do you think I should do?
Isn't it a nurse manager's job to help out when needed? So doesn't that included giving your primary night shift nurse a break and maybe take her shift for two nights????

Let me know what you think. I am a valuable employee to this company, and I really don't want to quit over something as stupid and childish, like not getting my way with my leave. But I have missed out on two very important events already this year