I think what happen to you is a common problem. When you are dealing with small fish lifting the rod tip catches the fish every time. But for larger fish you need to set the hook with a line pull. Simply point the rod tip at the fish and pull the line back. On a good size fish Ill do this two or three times. Once you have them on then I will raise the rod tip and set the hook again.

I hope this helps.

Island Fly