Release angle too low
This is the most common cause by far. A release angle under 30 degrees produces a flat, dying trajectory that looks fast off the hand but runs out of lift halfway through flight. Drill it with standing throws at a target marker set at 35-37 degrees, and let your body angle match the release angle rather than the other way around, since forcing the arm up without adjusting body lean rarely holds up at full speed. Most throwers need several weeks of standing-throw reps before the higher angle carries over into a full spin throw.

