Bhagavad Gita, Chapter 15, Verse 11

The Yogis striving for perfection behold Him dwelling in themselves; but the unrefined and unintelligent, even though striving, see Him not.

Even real, sincere effort isn't described here as automatically enough, a purified mind is named as the actual requirement.

Krishna doesn't say trying guarantees success, he names a specific real condition, purification, as what separates those who succeed from those who don't. Effort and readiness are shown as two different things.