That is the keyword I Googled for solution and no luck.
I’m too old to learn new source control mechanism like Git so I stuck on SVN with XCode.
It works perfect usually but sometimes it show “xxx is not under version control”.
For me, the reason is some special character in file name , in my case , the @ character.
I googled and tried to fixed this issue but no luck.
And today, finally, I solved this issue with one single step, simple and straight.
Open “Terminal” application, go to the project folder and keyin this
svn commit -m “Happy SVN”
And you’ll see all the files (including file name with @ character) are committed successfully.
I don’t know why the commit won’t succeed in XCode, but I’m happy that it works in command line mode.
Update on 2016/03/02:
Sometimes you have to make those @xx.png files the Add status (with A in the right) in Xcode first before you do the command-line svn commit.