Never took the telescopic legs apart but most telescopic type devices use a caming action. They have a disk/cam that rotates off center which binds the leg in position. If it isn't working it is for one of two reasons. One the cam doesn't get a bite on the inside of the out tube, so it never fully cams to a point that it is totally locked. Or the cam is worn down and can't cam enough for a secure fit.
I have a an extension pool cleaning pole that works the same way. On mine the off enter disk has worn down so it could not get a good bite on the tube. I took it out and re bored it a little more off center that worked for a couple of years but eventually I had to replace the disk.
Simple process. Just turn a disk the inside diameter if the tube. Then drill a hole slightly off center and attach it to the lower part of the leg. Slide it in the tube and you are good to go.
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