Has the awning fabric ever been replaced? If it has, perhaps the spring was unwound in the process. That happened when I replaced the fabric on mine a few years ago. When I rewound the spring, I didn't get it quote tight enough, and it exhibits the same symptoms yours does. I plan on doing this whenever I get around to it. Try extending the awning only about a foot. You'll find a small hole at the left end of the awning tube (as you're facing it). Insert a long hard object (like a long Allen wrench) into that hole and fish around with it until it fits into another hole at the bottom of the first hole. That locks the awning tube and keeps it from spinning (unwinding). put a piece of strong tape over the top of the Allen wrench to keep it from falling out when you turn the awning tube. (I know because mine fell out and the awning spring unwound.)
There is a tab on the awning roller end cap that fits into a slot at the end of the awning arm. Put a channel lock on that end cap and hold firmly onto it while someone else removes the screw that holds the tab into the slot. They will also need to support the awning arm while you carefully slide the tab out of the end of the slot. BE CAREFUL!! THE SPRING WILL TRY TO UNWIND!
Now, rotate the endcap toward the direction of the spring's resistance. It should only take one or two rotations to sufficiently tighten the spring. Next. carefully reinsert the tab into the alignment slot on the awning arm and reinstall the screw. Finally, remove the tape and Allen wrench from the hole in the roller end cap. (You may have to rotate the roller slightly by hand to relieve some of the tension that is binding the Allen wrench into the hole.)
Now, extend and retract the awning a few times to see how it works. If the spring needs more tension, you may need to repeat the process. If it doesn't help at all, you may need a new motor (right end of the awning) or a new spring (left end of the awning - as you are facing the awning side of the rig). Both can be found on line by doing a Google search for RV awning parts. I went through MarineandRVsupplies.com.
Hope your search for a solution is successful!