Glitched Minecarts

Everyone on my server is having an issue where when you click to get into a minecart, you are not fully seated in the cart. We are about a block above where the cart sits. After a few seconds, you start to slide down into the cart, but are still too "high up" in the cart. Down every track, we have half slabs to keep mobs off. It used to be that your eyes would be at that level and you wouldn't see the slabs. After a minute of riding on the tracks, the character becomes seated properly.