Quicklook stopped working with xcode's .m and .h file suddenly
You might consider simply using QLStephen which will add proper QuickLook support for all text files regardless of their extensions. It will also properly handle files without extensions (e.g. README, Makefile). What I like about it is that you don't have to maintain a list of extensions; QLStephen detects text files automatically. QLStephen is independent of XCode and is a QL plugin that you drop into /Library/QuickLook/ or ~/Library/QuickLook/.
The fact that I am also named Steven is purely coincidental.
Edit: Debugging QuickLook based on more info from OP:
Turn on QuickLook logging:
defaults write -g QLEnableLogging YES

Here is some reference logging output (written to Console):
Stock Xcode installed:
2014-04-29 2:46:08.172 PM quicklookd[19315]: 
    [QL] Thumbnailing file://localhost/Users/user/src/hello/main.m. 
    Content type UTI: public.objective-c-source
    Generator used: &lt;QLGenerator Text.qlgenerator&gt;

2014-04-29 2:46:08.173 PM com.apple.quicklook.satellite[19330]:
    [QL] &lt;QLThumbnailRequest main.m
            token: d45af7955eb9401bc7a49fd6652d1c3aa5cc4a8b;00000000;000000000000001a;
            com.apple.app-sandbox.read;00000001;01000003;0000000003e32849;
            /users/user/src/hello/main.m&gt;.
    Content type UTI: public.objective-c-source.
    Generator used: &lt;QLGenerator /System/Library/QuickLook/Text.qlgenerator&gt;

Third-party generator (QLColorCode) installed:
2014-04-29 2:51:40.458 PM quicklookd[19359]:
    [QL] Thumbnailing file://localhost/Users/user/src/hello/main.m.
    Content type UTI: public.objective-c-source.
    Generator used: &lt;QLGenerator /Users/user/Library/QuickLook/QLColorCode.qlgenerator&gt;

2014-04-29 2:51:40.459 PM com.apple.quicklook.satellite[19367]:
    [QL] &lt;QLThumbnailRequest main.m
            token: d45af7955eb9401bc7a49fd6652d1c3aa5cc4a8b;00000000;000000000000001a;
            com.apple.app-sandbox.read;00000001;01000003;0000000003e32849;
            /users/user/src/hello/main.m&gt;.
    Content type UTI: public.objective-c-source.
    Generator used: &lt;QLGenerator /Users/user/Library/QuickLook/QLColorCode.qlgenerator&gt;

Since things work for you when calling qlmanage from the command line, you might be able to compare the logs written to Console with the output from manually invoking on the command line and spot the issue. With only Xcode installed:
[user@macbook]/Users/user/src/hello$ qlmanage -p -d 1 main.m 
Testing Quick Look preview with files:
    main.m
[DEBUG] Registering &lt;QLGenerator Built-in 0x7fa08241eaa0&gt; for public.image
[DEBUG] Preview test for main.m -- file://localhost/Users/user/src/hello/.
        Content type UTI: public.objective-c-source
[DEBUG] Previewing file://localhost/Users/user/src/hello/main.m.
        Content type UTI: public.objective-c-source.
        Generator used: &lt;QLGenerator Text.qlgenerator&gt;
[DEBUG] Loading &lt;QLGenerator Text.qlgenerator&gt;
[DEBUG] Previewing file://localhost/Users/user/src/hello/main.m finished