2 Sam 21:18 There was again a battle with the Philistines
In his life there were many nations David battled with and you never hear of them disturbing David again but from the first battle with Goliath all through his life David has to battle the Philistines again and again.
There is a duality of deliverance. Some deliverance is immediate, total and final. Other forms of deliverance have to be continually fought for.
Interestingly, you never hear of David losing a battle with the Philistines but nonetheless he had to keep fighting them. As he fought them God gave him the victories.
Paul says in his writings that he had a thorn in the flesh. He asks God to take it away, to which God basically tells Paul “No. I wont take it away but I will show you My grace.”
God’s deliverance for us indeed can be instant and immediate but in other instances of our Christian walk God delivers us as we keep contending for His grace in our circumstances.