I have seen this a half dozen times.
Simple keyboard maintenace..
turn off computer and unplug keyboard from computer.
On the back of most keyboards is a couple of phillips head screws. Remove all of them.
This exposes the inside of keyboard. usually it's a film of some type clean this with windex. Many of these are plastic with a silver film on them if it's tarnished black anywhere clean with a pencil eraser. (note windex is an ammonia based product. Rubbing alcohol will do if windex is not available)
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