At the risk of sounding condescending: are you using the whole pack? I've seen new backpackers who didn't realize that their 60 or 70 liter pack has a divider about a third of the way up from the bottom, which separates the pack into a compartment for the sleeping bag (accessed by a zipper at the bottom of the pack) and a main compartment (usually loaded from the top.) They also don't realize that this divider can usually be zipped open or unbuckled, to open the pack into a single large compartment.

Not realizing this, they've tried to stuff everything into the main compartment, and put nothing in below, effectively turning a 70 liter pack into a 50 liter pack.