It's too long or short, too heavy or light. Not too many people tie their own leaders any more, but the fishermen who do, have a much greater understanding of how a leader works. Your fly rod is tapered also. All the power from your arm must go to the rod through the line into the leader to get the fly where you want it to go. If you have a bad connection anywhere from your hand to your fly, there can be problems.
All leaders have four parts.
The butt section is the longest where it resembles the fly line in flexibility and density. This makes the transfer of power from the fly line to the leader possible. The midsection or body joins the butt to be an additional length of low visibility monofiliment to continue the taper. The midsection joins the tip that is usually 12 to 24 inches long then joins the tippet that is 18 to 30 inches long. Leaders and tippets are based on diameter in thousandths of an inch.