See these instructions. Scroll down to view figure 12 - shows the leaf springs and the piece that should seat into the slide rails:
http://www.carid.com/images/reese/pdf/reese-30048-installation-instructions.pdfPull the pins from the hitch legs.
Roll the hitch forward in the bed.
Spray heavily with WD40 or some other lubircant toward the leaf springs inside the vertical legs
These have to be kept lubricated as they control the "pick" like points that enter the slide rails.
Make sure your hitch slides all the way forward and rearward - may have to do this by positioning the locking handle in the center position so it wont lock forward or rearward and manually slide the hitch front to rear from within the truck bed.
Be sure the rollers are well lubricated too. They are just heavy rolled steel - no bearings - and roll on the slide rails as well.