With the slide fully out go under and check the "rack" the row of teeth under the slide arm. Look for any missing or broken teeth also check the gearpack - the "roller with a gear" for any damage. Then look at the floor of the slide for damage.
Lippert has a procedure for lubricating the gearpacks. Then use a dry lube or a "slide lube" - I get mine at Walmart - and spray the racks and tubes. If you have no damage this may help.
2009 Seville VF32KS
2019 GMC Sierra 3500HD Crew Cab Denali.
6.6 Diesel standard box.