The /cast command is pretty picky when it comes to your spell name. In order for it to sucessfully cast a spell you must use correct spelling, punctuation, spacing, etc. The best way to guarantee you enter the right name is to open your spell book while writing the macro, place your cursor in the macro where the spell should be, and shift-click the spell in your spell book. This will enter the exact name of the spell including its rank like the following:

/cast Shadow Word: Pain(Rank 5)

You can safely remove the (Rank 5) from the macro and it will automatically pick the highest rank--assuming you want to, of course; there are many situations where it is advantageous to use a particular rank (like specific timing on a Warlock's banish). Beware, though... Some spells with parentheses in their name may need some coddling. For example, using shift-click on a Druid's feral Faerie Fire spell produces the following macro:

/cast Faerie Fire (Feral)(Rank 2)

If you remove the entire (Rank 2), the internal code that translates the spell name gets confused by (Feral) thinking it is a rank and the spell doesn't cast. You will have to change it to the following in order for it to work as expected:

/cast Faerie Fire (Feral)()

The empty parentheses are seen as the rank since they're at the end. Because they're empty, the parser picks your highest rank and casts the spell. You can also use empty parentheses for other spells, but with the 255 character limit, space is definitely an issue.