Genesis 15

The Lord taught -

"For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD.
9 For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts." (Old Testament | Isaiah 55:8 - 9)

As we read the Old Testament, the Lord's timing in keeping His promises are also not of man's. The Lord promised Abraham -

"..to make of thee a minister to bear my name in a strange land which I will give unto thy seed after thee for an everlasting possession, when they hearken to my voice." (Pearl of Great Price | Abraham 2:6)

One would think that the Lord would give Abraham this land permantly and immediately, but shortly after Abraham settled in the area there was a famine in the land and Abraham had to go to Egypt. After his return the Lord reveals to him that before that before He would again be given the land -

"Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
14 And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.
15 And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.
16 But in the fourth generation they shall come hither again..." (Old Testament | Genesis 15:13 - 16)

And then the Lord reveals why -

"...for the iniquity of the Amorites is not yet full." (Old Testament | Genesis 15:16)

The Lord loves all His children. His blessings always seem to be hinged on obedience and worthiness. The Amorites were not fully ripe yet. When they were, they were to be removed. Years later, the Hebrews were not worthy even with the promise of Abraham and they were removed. Speaking of the Israelites return to the promised land, Nephi taught -

"...after they had crossed the river Jordan he did make them mighty unto the driving out of the children of the land, yea, unto the scattering them to destruction.
33 And now, do ye suppose that the children of this land, who were in the land of promise, who were driven out by our fathers, do ye suppose that they were righteous? Behold, I say unto you, Nay.
34 Do ye suppose that our fathers would have been more choice than they if they had been righteous? I say unto you, Nay." (Book of Mormon | 1 Nephi 17:32 - 34)

It seems when the Lord gives promises, they are conditional upon righeousness, obedience, and the Lord's timing.