Nothing jumps out at me, but I occasionally get messed up using shift if it is keybound to something else. If I remember, I think SHIFT+1 through SHIFT+6 are bound by default to action pages. I may be wrong, but that is usually the first thing I check to see if that key (with modifier) is bound to something else.
What Roogna said. Keybinds take priority over macro modifiers and by default SHIFT 1-6 are bound to Action Page 1-6. You’ll need to unbind them to use them as macro modifiers.
Also, if you’re only trying to cast 1 spell per press you could only be using a single /cast command.