A lot of times they leak where the keel touches the ground when you drag it from spot to spot. You won't be able to see through in all cases but mine did it a couple years ago and I could push my finger through what looked like solid plastic, right on the keel, right at the back of the yak.
What you do is get a 5-gallon bucket and use a hacksaw or other fine blade and cut strips of plastic, maybe an inch thick at the widest. Then you use a heat gun like the type used to strip paint. I actually had to buy one of those it was like $15 or so.
The key is to PRACTICE about 4 or 5 times before doing it to your kayak. Practice on the bucket. Heat up the spot you are going to "patch" a little, it will start to liquefy, heat up the strip of plastic, and basically rub it on the patch spot... be careful because it will go from plastic to liquid in a hurry... and your keel can cave in... I know from experience
It's not that bad though, I'm not very handy, and I was able to do it without hurting anything. I practiced a couple times then went slow. I had to build up material, let it cool, then go again a few times.