If you’re regularly hooking into river trout but your “landing rate” is dismal, you’re not alone. By the way, it took a lot of hard work and practice on your part to get where you are now so congratulations. 

Assuming you’ve presented the correct fly, the fish takes your fly, and you’ve managed a good hookset, you’re still miles away from getting that lunker into your net. Inexperienced anglers are always bewildered when the fish is suddenly off their line. It still happens to me but far less.