Multiple times I would reach a new section of a level only to be stuck for upwards of 10 minutes without any hint of how to proceed. (stevivor.com)
Also, sometimes players start in completely different sections of the level, completing their own area before meeting up sometime later. (playstationlifestyle.net)
Higher sections of the level will then become inaccessible, and as such it's necessary to complete those portions first. (acid-play.com)