Shloka 15.10 said the wise can see what the deluded cannot. Now Krishna adds an important nuance: even effort alone is not enough. Yogis who have turned inward and refined their inner being — they see the self seated within. But those whose inner self remains unrefined, even if they try hard, cannot perceive it.
The key word here is 'akritatma' — one whose self is un-shaped, un-prepared. It is like an unfired clay pot that cannot hold water. The clay is there, the shape is there, but it has not yet been hardened by the fire of inner discipline. Effort without that inner maturation falls short.
This is not a harsh judgment. A seed that has not yet sprouted cannot bear flowers — but the potential is fully there. Effort waters that seed. Over time, the inner preparation catches up, and the vision opens.