Depending on what your issue actually is, this may not offer a solution as such: But (if you are unaware) there's plenty of instruction here.
This was a defect introduced in JMP 15. You can achieve the same behavior using gridlets, as described in the documentation pointed to by @ian_jmp (thanks).
This defect has been fixed in JMP 16, but the preferred method for modifying content in Hover Labels going forward is to use gridlets.
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