In verse 4 we see that though land already BELONGED to Ephraim and Manasseh (sons of Joseph), they had to actually take it. Similarly, all of the spiritual blessings that we have are truly ours, yet we still have to claim them. Forgiveness is offered, but it must be received. Salvation is available, but it has to be requested. And yes, sanctification is guaranteed, but it takes our active participation for to be realized. How many things has God given to us, but we don't benefit from them because we don't take the initiative (James 4:2).
Notice that the tribe of Ephraim did not completely conquer their Canaanite enemies. God's instructions had been clear. Total conquest was the demand, but Ephraim settled for a measure of domination. It would certainly cause problems later. Even so in our own lives, there is no substitute for the vigorous devotion that God expects from His children (Romans 13:14).