Pekin Duck Limping

Hi, I have a small flock of four pekin ducks. They are very healthy and happy except for one. I have noticed her having a problem with her left leg. She limps around ,but she has done this since she has been an adult. I've examined her and she doesnt appear to have any visible problems and her leg doesnt seem to be tender to touch. However she continues to have a hard time keeping up with the other ducks. I put her up for a week to see if she had just possibly sprained it and was struggling. Unfortunately she didnt appear to do any better. I need advice as to what I should do. If this is possibly a birth defect that happens from time to time but doesnt really cause her any pain I will leave her be. But I am worried she might be suffering and I am just not aware. If that is the case I will put her down. I need help please, any advice would be greatly appreciated. Thank You