And the Amorites forced the children of Dan into the mountain: for they would not suffer them to come down to the valley:

Wesley's Notes

1:34 The valley - That is, into the plain country; which was the occasion of that expedition for the getting new quarters, of which we read Josh 19:47,48 and chap.Jud 18:1 - 31.

Matthew Henry's Concise Commentary

1:21-36 The people of Israel were very careless of their duty and interest. Owing to slothfulness and cowardice, they would not be at the pains to complete their conquests. It was also owing to their covetousness: they were willing to let the Canaanites live among them, that they might make advantage of them. They had not the dread and detestation of idolatry they ought to have had. The same unbelief that kept their fathers forty years out of Canaan, kept them now out of the full possession of it. Distrust of the power and promise of God deprived them of advantages, and brought them into troubles. Thus many a believer who begins well is hindered. His graces languish, his lusts revive, Satan plies him with suitable temptations, the world recovers its hold; he brings guilt into his conscience, anguish into his heart, discredit on his character, and reproach on the gospel. Though he may have sharp rebukes, and be so recovered that he does not perish, yet he will have deeply to lament his folly through his remaining days; and upon his dying bed to mourn over the opportunities of glorifying God and serving the church he has lost. We can have no fellowship with the enemies of God within us or around us, but to our hurt; therefore our only wisdom is to maintain unceasing war against them.