Why weren't the Israelites able to conquer some of the cities in the promised land? [Spirituality & Religion]

God promised to give over all their enemies to them as long as they remained obedient.
And aside from an unfortunate event in the city of Ai which was rectified by killing the guilty party and his entire family, they do.

So why are there a few instances where they Israelites were unable to drive the enemy from their domain? The Canaanites, Geshure, Maacah and the Jebusites apparent;y could not be removed and continued to live among the Israelites.

[c=#BF0000]*Never mind, this is answered in Judges[/c]