Naturally the more one-pointed the aspiration the swifter the progress. The difficulty comes when either the vital with its desires or the physical with its past habitual movements comes in—as they do with almost everyone. It is then that the dryness and difficulty of spontaneous aspiration come. This dryness is a well-known obstacle in all sadhana. But one has to persist and not be discouraged. If one keeps the will fixed even in these barren periods, they pass and after their passage a greater force of aspiration and experience becomes possible.
You are finding it still difficult to bear the interval periods when all is quiet and nothing being done on the surface. But such interval periods come to all and cannot be avoided. You must not cherish the suggestion that it is because of your want of aspiration or any other unfitness that it is so and, if you had the constant ardent aspiration, then there would be no such periods and there would be an uninterrupted stream of experiences. It is not so. Even if the aspiration were there, the interval periods would come. If even in them one can aspire, so much the better—but the main thing is to meet them with quietude and not become restless, depressed or despondent. A constant fire can be there only when a certain stage has been reached, that is when one is always inside consciously living in the psychic being, but for that all this preparation of the mind, vital, physical is necessary. For this fire belongs to the psychic and one cannot command it always merely by the mind’s effort. The psychic has to be fully liberated and that is what the Force is working to make fully possible.
Ref : Letters on Yoga Vol. II