Yolanda,
What you have there is two questions... one simple technical point and one rather complex issue. Let me start with the simple one:
Generally in Mambo, the leg that is in the process of receiving weight is bent. So when you first close your feet on count 4, the closing foot should be bent, not straight. It won't straighten until the end of count 1, just as the next leg bends and begins to move to its next position.
Now for the tough question... Is this mistake causing you to lose? Well, it may be costing you a point or two here or there, but even if your knee action weren't entirely correct, it's still possible that you could give quite a convincing performance. There are so many different qualities that make a good or a poor dancer that you can't attribute your success or failure in the marks to just one. If you're having difficulty getting marked well, it's probably a combination of many different elements together, many of which you may not even be aware of yet.
You also have to remember that you're not being judged on a concrete scale. You're being compared relative to other dancers. So if you get marked poorly by a judge, it doesn't necessarily mean they don't like your dancing. It just means that they liked the other dancers better.
Since I can't watch you dance, here's my advice: Keep studying with your instructor, get the occasional coaching lesson with a professional coach, and practice consistently. Follow this formula, and I guarantee you will get better results with each competition.
Good luck!
Sincerely,
Jonathan Atkinson
