I assume the key is released? Could be a contact problem in the keyboard mechanics. Open up the keyboard section and clean the mechanics. This is wise to do on regular basis, especially if the synth is in dusty conditions. Also check if there is any physical damage on the low-end keys.
I bet it will start to work just fine after the clean up. There is just some grit below the rubber mat that interferes* with the contacts.
*First I thought to write "fucks up", but I decided to stay classy
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