Rather late to this party.... but in my experience there are two main reasons why people fail at MLM:
1) A non-duplicatable training system. In our business we have a 3 minute video to pique people's interest, and then (if interested) they are passed to see a business overview presentation, finally to an "expert" to answer questions. So the new distributor is NOT presenting the business to people. We are using a duplicatable system, and the prospect thinks "yeah, I can show people two videos"
2) People get fed up of buying from the same range of products month after month, when they haven't used up or sold their previous month's supply.