Make sure you are pulling/pushing reasonably straight both up/down and side-to-side. Our pup was not a HW but we found this would be the case if we weren't paying enough attention. It worked better for us with one person as one of us would generally 'get ahead' of the other. Once the bed is further in the rails it was less of an issue.
2016 Roo 21SS, 2016 F150
2015 Rockwood 2514G (pup), 2015 Fd Expl,
2003 Coleman Utah (pup), 2007 Fd Expl