Hi there, is the Polar under warranty? If so, that would be the first port of call. Reason being, you wouldn't want to void your warranty if you still have one.
Otherwise, if you feel able to take a look inside, there is a similar recent (ongoing) thread to your query with pics, etc.:
The pics are for a Virus Indigo, but I'd imagine the Polar would be quite similar. If or where it varies, you would have to use your intuition.
If the ribbon cable is inserted correctly, and you still have the problem, then it will likely be the key-contacts.
I, personally, haven't opened up a keybed yet to see what's inside (as I have had no need to), so unfortunately I don't have specific pics for doing that.
As a guess, after removing the keybed from the Polar, I think you would need to remove the black plastic cage underneath the keybed, before opening up the PCB that houses the key contacts.
The video on the thread I linked to above will show you how to clean the key contacts themselves.
It shouldn't be too hard I wouldn't think. But it will require due care not to damage the rubber key-contact strips (even though they would likely be very cheap to buy, even if you did so).