Question: XCode 6 Beta 5 and autocomplete stopped working. I thought problem in Beta 5 but I've reinstall to Beta 4 and it's still not work but it did before. I check preferences for "suggests" and I tried to delete Derived Data but it doesn't help

Answer: 
This happened to me recently because I had a /\* \*/ style comment after the @end of my @implementation.

Not sure why that would break autocomplete but moving the comment into the implementation fixed autocomplete.

There seems to be a lot of reasons autocomplete breaks. Just adding to the list..
I was having this problem. 

This is what worked for me. 

1. Clear derived data.

Window -> Organizer -> "Select you project" -> Delete Derived Data. 

2. Clean the project
3. Quit Xcode ----- this is very important. You might have to restart you computer.
4. Open Xcode again

Hope this works for you. This worked on 2/9/2015