Jamieson, Fausset and Brown on Haggai 2:19

19. Is the seed yet in the barn?--implying, It is not. It has been already sown this month, and there are no more signs of its bearing a good crop, much less of its being safely stored in the barn, than there were in the past season, when there was such a failure; yet I promise to you from this day (emphatically marking by the repetition the connection of the blessing with the day of their obedience) a blessing in an abundant harvest. So also the vine, &c., which heretofore have borne little or nothing, shall be blessed with productiveness. Thus it will be made evident that the blessing is due to Me, not to nature. We may trust God's promise to bless us, though we see no visible sign of its fulfilment (Ha 2:3).