Of what was provided:
i. Not calling out/defining Trump early. Cruz and Rubio literally ignored (or sometimes mildly supported) Trump on the debate stage
ii. Not coalescing. To be fair, there was no clear person to collapse upon. Rubio was too much of a greenhorn, Kasich is too weird (despite LOLfric's obession of him), Cruz too factional, Carson a clear con artist, Bush emasculated, etc. etc.
If Rubio hadn't been a moron and you know, actually developed a ground game and made a clear effort to win a state (Iowa or South Carolina) and executed upon it, he would've had a glide path to the nomination.
Rubio never being able to pick an early state that he could actually win was one of the most incomprehensible decisions of any candidate in this entire race, and it was patently obvious to anyone who had seen the 2008 Giuliani campaign that it was likely to end disastrously for him.