The tests stop executing after first wrong answer encountered.

If you don't receive any error message, then it fails silently.

Ermm, that's what random tests do. Are you sure your code works with capital letters too?
818 others didn't have a problem with it in C.

I think this kata is broken in C. Every time I make an attempt it changes the number of tests that I pass. Sometime is 2, sometimes 6 or 13 tests that I pass.

FYI:

• The "Issue" label is intended for reporting problems with the Kata itself (e.g. poor description, poor test coverage, incorrect assertions preventing correct solutions from passing), not problems in your solution for whatever reason (bugs, misunderstanding of description etc.). Such comments (asking for help to solve a Kata) should be labelled as "Question" instead.
• When asking for help in a Kata, make sure you format your code properly so fellow Codewarriors do not need to strain their eyes to review your current solution. Also, it may help if you state the language you're attempting the Kata in - it may not always be apparent to the people reviewing your code (though in this case I'm assuming that you're attempting this Kata in C?).

Cheers :)