User blog comment:Omega natsu2/Episode 179: Gray vs. Rufus, Review./@comment-288216-20140426233755/@comment-7418318-20140427074851

"@Nick, yeah i know. That's why i said it's a failure in imagination. ( ._.) This imagination thing doesn't live up to their expectation."

It was never said that he can freely conjure everything he imagines, that'd be "Arc of Embodiment". Just because he can imagine a tripple combination spell doesn't mean he has the ability to cast it, so I'd chalk it up as a "failure" in ability rather than imagination, though failure isn't the word I'd use.

Considering how he easily beat all other opponents in the GMG, maybe he just didn't train with such combinations because they were never necessary and he could do it. So maybe he just can't use tripple+ combination yet but might learn to do it through more training. Training for which he naturally wouldn't have the time in the middle of an intense fight, where a single misfired spell can easily become the difference between victory and defeat.