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A. you might have a defective USB cable... try another one.
B. there might be dirt gotten into the USB jack on the Olympus recorder. If so, it can be brushed out with e.g. a clean, but dry, old toothbrush. (It must be dry. Wet could hurt the circuitry, you know.)
How old is the voice recorder in question?
Then it may be 'broken', quite simply.
Try it first in a different USB port - or several. Wait at least 4 minutes before removing it from a USB port, and of course, Stop the device in your computer's operating system, to avoid damaging it, before removing it.
It may be 'broken' in the non-mechanical sense. A static discharge (invisible, and when low-intensity enough, impalpable) probably got into the USB port and ruined the USB interface circuitry. (Static discharges may be avoided by never wearing a garment like a worsted sweater, or any type of woolen upper garment - and/or not keeping anything like your recorder in the pocket of a woolen garment. Wool as a fabric tends to accumulate static electricity.)
Or it might just be, that examination with a good magnifier would reveal tiny cracks in the places where the USB jack attached to the internal circuit cardlet of the recorder.
But since all things that could get it working have been tried, I think it might need repair-servicing.