There are only three fundamental obstacles that can stand in the way:
1) Absence of faith or insufficient faith.
2) Egoism—the mind clinging to its own ideas, the vital preferring its own desires to a true surrender, the physical adhering to its own habits.
3) Some inertia or fundamental resistance in the consciousness, not willing to change because it is too much of an effort or because it does not want to believe in its capacity or the power of the Divine—or for some other more subconscient reason. You have to see for yourself which of these it is.
Ref : Letters on Yoga Vol. III