Cooked potatoes can go a bit gloopy when they are blended.  This is the reason why it is best to make mashed potatoes with a ricer rather than in a food processor.  The ratio of potatoes to leeks is important here as it will prevent the soup texture going gluey from the blended potatoes.  So don't be tempted to add more potatoes into the mix! It also helps to only blend for just the amount of time necessary to get a smooth texture and no longer.