The 3 noteworthy late entrants in the last 3 elections were General Wesley Clark, Fred Thompson, and Rick Perry. All 3 jumped in the race late in the process and looked like rising stars but flamed out quickly. The fact that they entered late had nothing to do with it though. Fred Thompson flamed out because he thought he was just going to be handed the nomination and proceeded to run one of the laziest campaigns of all time. Rick Perry admitted previously that he actually decided he was going to run for president 2 months before he jumped in in August. Perry's bad results weren't based on the fact that he entered the race late but that he was a terrible candidate. Apparently running for lower offices in America must not be that tough because Perry was viewed as an unstoppable juggernaut in Texas yet once he stepped onto the national stage he looked like an amateur. He's certainly in good company. Herman Cain, Sarah Palin, and George W Bush among others all got a rude awakening when they found out that in a 24/7 media environment, you can't just say whatever you want without thinking or knowing things and get away with it.
As annoying as Mitt Romney can be, he's at least running for president with the kind of skill and preparedness that Barack Obama did. Even Hillary Clinton made an amateur mistake in 2008 by not considering how important it was to rack up delegates in the caucuses and it led to her downfall. You can't just wing it when it comes to running for president and thus there's nothing inherent about being a late entrant that makes it tough to win the nomination, it's that you need to be very very very prepared and someone who jumps in because his friends goaded him into it likely hasn't put in the necessary work to handle it. People complain about the primary process but the last few elections, the media, the parties, and the people have actually done a good job of weeding out the candidates that weren't ready for primetime.