Canon and HP laser techo checking in here,
This is a fairly common problem among the Canon LBP-600 series and HP 5/6L series (same engine) and the Canon LBP-800 and HP LJ1100 series (these also share the same engine).
Essentially there are two contributors to the problem. The first and definitely most common is the separation pad in the paper pickup unit. This consists of a little L-shaped bit of plastic with a gummy rubbery bit attached to it. Typically what happens is the gummy bit becomes worn and slick over time, gaining a somewhat shiny appearance. The pad needs to be replaced periodically.
Another contributor is worn paper pickup rollers. In the LBP-800 and LJ1100 series this is easy to replace. In the LBP-600 and LJ5/6L series it’s a real pain in the arse to replace.
Your best bet is to replace the separation pad in the paper pickup unit. It’s a fairly straightforward job, but if you are not familiar with printers, I’d suggest leaving it to a professional. On both these printers it will require removing most of the covers and removing the paper pickup unit.
If you were in my neck of the woods, I’d do it for you in exchange for dinner. But since I’m not, take it to your local printer repair place. They’ll charge you anything from 30 minutes to 1 hour worth of labour, plus parts (the sep. pad is cheap, maybe $5-10). It’s unlikely to be covered under any sort of warranty as it is a part affected by wear and tear.
Hope this helps,