It sounds like it may be a case of compressed battery tabs.
Please remove the battery cover and bend the tabs outward to make sure that they're making proper contact with the batteries inside.
While the cover is off, press down on the batteries inside the unit. These should feel like they're against a slightly springy surface.
If they feel like they're against a solid surface, these tabs may be compressed.
It's worth removing the batteries and looking inside to check for corrosion as well.
If the battery cover tabs don't make a difference and the batteries feel solid inside, pull the internal tabs as well. Here is a short video about making a simple tool to pull the internal tabs: https://drive.google.com/file/d/1NEM8QImsQY8BweBtmH5jHbnR6ziqKrVv/view?usp=sharing
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