i can say without any reservations it's not the lenses. whatever you use won't net you 100% keepers, but unless something silly is happening you should be getting something north of 80%. i also used to shoot sports and still do with my kids. clearly sporting events requirements vary, but i've never seen any sporting event like a wedding reception. it's good exp though, just different, like all the other niches in photography. in my exp i would say that if my 5dmk3 can get a lock on something at a wedding, it doesn't really lose it. i really only struggle when it's so dark i can't get anything out of it. then i switch to MF and shoot it up. sometimes it is just silly dark. i'd like to know what aperture values you were using, and maybe a rough estimate of shooting distance. either that was just to much of an ask, your 5d settings are funny, or it's defective. you liked what the 1dx could do, was that a wedding, or people running around on a sunny day?