I used to have the same problem, after a year or so my keel rollers would seize on their shafts and the rollers would collapse in on themselves. This resulted in all the weight being placed on the side bunks, making the boat difficult to push off and retrieve especially on a shallow ramp. My boat is a heavy whaler clone, and is meant to be supported along the keel. I now have seven keel rollers, three pairs and one single one at the front. Over the past two years, I have had no problem launching or retrieving the boat, the rollers still turn easily and show no sign of any wear or flat spots, and my hull still sits about a quarter of an inch off my side bunks. At this point I am very pleased with them. I used stainless shafts and retaining caps and greased them with marine grease before installing them. For every good thing there is usually a downside, and it is true in this case as well. They are not cheap and I had to get them from the USA as I could not find them anywhere in Canada. Steveston Marine said they had them but they were just a cheap imitation and did not have the inner support. This is the website if you are interested.
http://www.stoltzind.com/
http://www.stoltzind.com/product.html
