The first things to check are the battery tabs. The tabs on the cover may become compressed after repeated battery changes.
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 you're still having trouble please feel free to email us at firstname.lastname@example.org.