I can only suggest what I'd suggest to anyone with a sticky key problem. Hold the keyboard upside down and give it a good shake in all directions to dislodge biscuit crumbs etc. If that doesn't work, you could try gently prizing the fault keytops off and checking for gunk underneath them. If it were a separate keyboard, you could put it in the dishwasher - but I'm not sure that's appropriate on this occasion.
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