I've found whichever shaman gets invited first does the most DPS and whichever one is the last invited into the group does the least. This is with the same macro's, same spec and same gear.

I reset the damage meter when I invite toons to start my dailies, and invariably, the shammy in the F2 slot on the paladin is top damage, while the shammy in the F5 slot is bottom damage.



Sometimes a target will resist a spell.
The macro will advance, because it did cast the spell.
With a spam macro, there's not much you can do about that.
You'd need to manually cast your shocks etc.

Once you are hit capped for the content you are facing, your spells should hit all of the time.