It is hard to know whether you want to know what shows that he has failed or what causes him to fail.
As far as what shows that he has failed, we can see that he has failed because none of the kids except Simon and Piggy are really willing to keep on helping him build the huts. The rest of them do not stay focused, and he is the one who ought to be keeping them that way (if he is a leader).
As far as why why he has failed, I think the only thing we can really say is that he is a bit whiny and not very positive. When he talks to Jack he seems like he is complaining. He ends up talking about how he doesn't actually like working. This attitude would not be very inspiring, I think.
While Ralph looks the part of a leader, he does not understand how to motivate the boys or organize them.
By Chapter Three of Lord of the Flies, Ralph has called several meetings, at which the need for the rescue fire and the necessity of building shelters is stressed. These needs Ralph communicates to the others; however, he does not know how to motivate the boys, so they abandon the construction of shelters because it is frustrating work; often the shelter collapses during its construction. Most of the boys go on hunting expeditions instead of struggling with the construction of a shelter because there is excitement in hunting as well as immediate gratification if one of the pigs is killed. The killing appeals to the boys' baser natures, and they succeed in procuring food as well.
When Ralph argues with Jack, he does not convince Jack of the exigency of keeping a rescue fire burning and of constructing shelters:
"I was talking about about smoke! Don't you want to be rescued? All you can talk about is pig, pig, pig!"
"But we want meat!" Jack retorts.
"And I work all day with nothing but Simon and you come back and don't even notice the huts!"
"I was working, too--" Jack claims.
"But you like it! ... You want to hunt! While I--"
Since their hunger and desire for meat and the thrill of the hunt are priorities for the boys, shelter-building and keeping a fire going take a back seat.
In the book Lord of the Flies Ralph can not get the children organized enough to build shelters. He is frustrated at the lack of help and the shelters fall apart easily. He and Jack have a conflict over the need for shelters verses the need to kill a pig.
Ralph is also focused on getting and keeping the rescue fire. He fails to recognize that until the children's basic need of hunger is met, they are not going to concentrate on anything else.