photos won't restore from time machine backup?

I had to restore my imac from my time machine backup. I did it once just by restoring from backup, then 2 weeks later I had a problem again and so I reinstalled the os and then restored from my time machine backup again. Everything is working great now, except my photos library. First it wouldn't even open, just said there was some problem with the library or library couldn't be found. So then I tried to go into the time machine and just restore the photo's library. I've tried it 3 times from 3 different dates now and everytime it goes through the process and almost finishes copying but then eventually says it couldn't be completed due to an unknown error. Help? I have years of photos now missing!