Why does my caramel sauce fail?
It sounds like you may be having temperature control issues.  If it's not dark enough, keep it over low heat for a bit longer.  If it's hardening/burning, it's probably caused by one of the following:

1) your stovetop (if burner isn't turned low enough or burner is too large and overheats sides of pan) 

2) your pan (easy to burn things if your pan isn't sufficiently thick-bottomed) 

3) lack of attention during the carmelization process (you need to move the sugar towards the center once carmelization begins to prevent burning at the edges, but not stir it too much).

You might find these extremely detailed directions helpful for troubleshooting.