As
@poor but proud pointed out, you are just clamping a large nut down on the rail, if its a calm day pull open awning, remove the grub screw stop in the groove of the bottom arm so you can slip out the top rail, then swing the lower arm out to vertical, hold out with guy rope... you can then split the top rail into two and check out the slide lock to see if thread stripped or chewed...................
To get a good tension when you open the awning and set the top bar in poition, move the awning lever to close (holding barrel at same time, then pull down to apply tension where both top and bottom rails meet, when nice tension on awning do up the lock nut, do same to other side then loop guy around end of top rail passin over the the barrel junction then down to spring/peg ........... this should apply enough tension so that if your lock is poor it should stay in position and awning shouldn't open anymore ..... Of course if you are adding rails you have to jiggle the position so holes in the correct spot, just a bit of practice and it works easy.