SB 10.20.36

girayo mumucus toyaṁ
 kvacin na mumucuḥ śivam
yathā jñānāmṛtaṁ kāle
 jñānino dadate na vā
Synonyms: 
girayaḥ — the mountains; mumucuḥ — released; toyam — their water; kvacit — sometimes; na mumucuḥ — they did not release; śivam — pure; yathā — just as; jñāna — of transcendental knowledge; amṛtam — the nectar; kāle — at the appropriate time; jñāninaḥ — experts in spiritual knowledge; dadate — bestow; na — or not.
Translation: 
During this season the mountains sometimes released their pure water and sometimes did not, just as experts in transcendental science sometimes give the nectar of transcendental knowledge and sometimes do not.
Purport: 

The first part of this chapter described the rainy season, and the second part has been dealing with the autumn season, which begins when the rain stops. During the rainy season water always flows from the mountains, but during the autumn the water sometimes flows and sometimes does not. Similarly, great saintly teachers sometimes speak expansively on spiritual knowledge, and sometimes they are silent. The self-realized soul is closely in touch with the Supreme Soul, and according to His desires a competent spiritual scientist may or may not describe the Absolute Truth, depending on the specific circumstances.