Matthew Poole and continuators on Psalms 99:7

7 He spake unto them in the cloudy pillar: they kept his testimonies, and the ordinance that he gave them .

i. e. To some of them; for the expression is only indefinite and therefore doth not necessarily reach to all of them: to Moses frequently; to Aaron, Exod. 19. 24. and 33. 9, 10, 11. Numb. 12. 5. And for Samuel, he answered him if not by words yet really and by his actions, thundering against the Philistins, 1 Sam. 7. 9, &c. which supposeth a Cloud, if not a Cloudy Pillar.

This is added not onely for their commendation but for the instruction of the Israelites to teach them that God will not hear the Prayers of them who do not keep his Commandments.