OK, here's the deal: "agile" *cannot* fail!
Here's why "agile" cannot fail: it is a set of tools that can be adapted to your needs. You may do better or worse with them. In that sense you may fail to benefit from them, or you may simply prefer not to use them. Or you may benefit from them. In either case it is you suffering or benefiting, not "agile".
If you think "agile" can fail, there is a different problem to talk about.
But "agile" cannot fail in the same way "hammer" cannot fail. (Thanks, Ed, for the analogy.)