Take a close look at the sails on boats that are sailing close to you... and, to your own sails. Are the sails smooth and round? Or, are there creases and stretch marks? Are the sails way too round? Here are a few photos of old and inefficient sails that I've snapped over the years.
This main sail has many creases and folds. It cannot move the boat effectively.
This old genoa jib has been stretched out badly at its clew corner.
This mainsail has been stretched out way too much along the leech edge. Probably from too much sheet tension in strong breezes.
This mainsail is blown out. Notice how the forward edge of the battens creates an edge along the line between the battens?
This mainsail is old and soft. It won't drive the boat well at all.
Here's another well stretched mainsail.
Yes, these sails will move the boat... but not very well. And, they will probably heel the boat over much more in the puffs than a newer sail would.
These sails are nice and smooth and drive the boat along nicely - even in light breezes.