I think of it the other way around, actually... spells don't work for you, you work for the spell to work. Spells aren't scientific formulas* that will work for anyone, any time... if you don't have a connection with the universe at some level, then nothing will happen.
If you do have a deep connection with the universe on many levels, then everything can happen -- even Harry Potter spells, because the words express intention.
* Except maybe some ceremonial magicks -- ancient mages got that down to a science. o_o