[Q] I cant get updates on my m7 topic

So basicly i unlocked my bootloader flashed custom rom(cyanogen mod)i was boren from cyanogen and back to stock with my recovery backup that i made before i flash the custom rom...Today i got update to install lolipop,downloaded the file it asked me would i like to instaal it so i click install..The phone reset(like always when i update)but it went to recovery and the update didnt start..So i was thinking hmm maybe it wont update becauuse the bootloader is unloceld so i relocked the booloader and when i am trying to install the update it goes to fastboot...So i dont know why i cant install it
--Does anyone have idea that is the problem how to solve it?

-Thanks a lot