This one comes from underxenith and I wanted to add it at the top here. I just got hit by this bug and his solution worked for me. If you have alot of neighbors (like me) it's very labor intensive but it worked.
1) Go to "My Neighbors" tab in the game
2) Carefully check each neighbor listed is the game is also in your G+ circles
3) If you find any neighbors not in your G+ circles, remove them from your "My Neighbors" list
I had one that wasn't in my G+ circles, tried to re-add him and the his G+ page said "Profile not found."
Hope this helps as well.
Ok, update on the missing neighbor issue. I found another problem with neighbors that will cause your "Play" screen neighbor bar to be empty (except for Rob).
1) In G+ open your Circles tab
2) Click on "People in your Circles"
3) Scroll through and look for a envelope icon on any of your G+ Circle friends (that icon means they don't have a G+ account, but you can share via email) Note those friends.
4) Open ZombieLane and click on the "My Neighbors" tab
5) Remove any neighbors from step #3 above.
6) Reload ZombieLane
Tested and worked for me when I was hit with the no neighbor bar bug again.
Note: If you don't have your some of your neighbors in a current circle, to fix this bug you will need to re-add them temporarily to your circles and then check for the envelope icon. Thanks to a friend of mine pointing this out.
Hope this helps!