The extent of the territory given these two tribes was not sufficient for them, in their opinion, so they asked Joshua for more land. He dealt with them very diplomatically by complimenting and encouraging them ( Joshua 17:18). Their complaint seems to have sprung from a spirit of carnality (cf. Joshua 17:16). [1] They did not share Caleb"s spirit ( Joshua 14:6-15). Evidently they counted as their land only the parts that were then easily accessible to them. They discounted the forested areas that needed clearing and the parts dominated by the Perizzites and the Rephaim ( Joshua 17:15). Joshua assured them that when they subdued their inheritance it would prove adequate for them ( Joshua 17:18). [source][source][source]
"The purpose of inserting this episode at the conclusion of the description of Joseph"s portion may be to alert the reader to the fact that the promised land, if it is to be possessed, requires the activity of the tribes, who must not be deterred by the threats of Canaan"s superior military force." [2][source]
The writer of the Book of Joshua noted carefully the failures of the tribes to drive the Canaanites out of their territories, as well as their successes in doing so (e.g, Caleb). The extent of their occupation of their land depended on the extent of their ability to annihilate the Canaanites by God"s power. [source][source][source]