It is difficult to point to specific mistakes that Ralph makes but there are a number of things that perhaps make it harder for boys to follow him and instead leads a majority of them to follow Jack and lead the island to greater brutality and murder.
At first, Ralph seems to be drawn to making a society of law and order on the island. He wants to set up shifts to keep the fire going, wants the boys to have orderly meetings, etc., but he also demonstrates that he shares some of the same desire to abuse his power when he makes fun of Piggy and doesn't always follow through with Piggy's suggestions.
Perhaps he lacks the ability to fully commit to his ideal and the boys sense this and quickly move to follow Jack who shows no hesitation to pursue the course he feels is right. This inability to fully commit is demonstrated again near the end of the novel when he still contemplates joining Jack but then finds out that Jack is going to kill him.