Well, I've been swimming for about a year now and I'm not too bad at it, except for not being able to kick hard and fast. I can do a 50 in about 27sec., but that's without being able to kick fast, my arms do all the work. I've been looking online and asking my coach about the problem and everyone is telling me the same advice: hips up, butt out of water, and legs straight. I've been trying to do these things, but I just go very slow and I become worn out just doing 25 m, usually I have to do a couple breast-stroke kicks just to keep going. My teammates have also been telling me that I am doing all these things, I'm just not moving like they are.
Can anyone please give me some advice as to what I'm doing wrong, I'm not in bad shape so I don't understand why some of my teammates who are bigger than me can kick faster than me.