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On the back plate, turn the focus motor leadscrew clockwise with your fingers and check that it reliably lifts the focus lens. The leadscrew engages in a threaded brass insert in the plastic piece holding the focus lens.
On a well worn camera the thread in the brass piece can fail resulting in a failure to focus. Unless you have access to watchmaker's machine tools, the only solution is to obtain a spare from another lens with a different fault.