And is your phone rooted (or has been in the past) or running a custom ROM? Those will also stop an update from completing (but not from starting if one is available).
Also, if the phone was bought from one carrier, and is now on another carrier, you won't get an update, you'll have to do it manually. (Read about 4.4 carefully before you decide. SD cards are no longer "free to write wherever you want" - an SD card is more like internal storage since 4.4. And Lollipop seems to have bugs on just about every device.)