How should I fix an electric shower pull cord that doesn't work anymore?
I've never seen an electric shower (looks to be a European thing?) but based on the diagram in that article you've got electrical contacts near moisture. Corrosion is quite likely and would cause intermittent problems like you describe. Turn off all power to the device then open it and check for "crud" on any of the electrical terminals. a fine brass-bristle brush can help remove it, or even a Scotch-Brite scrubbing pad.