I have a pair of one-year-old JBL 500BTs which had this exact same problem. The contacts between the board and the speaker consists of a pair of springs and over time, one or both springs may lose that contact.
Disassembly is as described by another poster, there are three screws under the foam and then the speaker can be gently twisted out of its enclosure.
Inspect whether the springs are sat at the same level and using a small set of pliers or other appropriate tool, you can gently pull the spring such that it can make contact when the speaker is seated.
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