It's batteries, batteries, batteries. I check the voltage on batteries before I install them into the earwig. I have found batteries that read as low as 1 volt, fresh out of the package. These are the ones that last a few minutes and then go into hash. Don't bother putting anything reading less than 1.2 volts in. If you find batteries reading 1.4 volts, your golden. I rarely find batteties that test at the 1.5 volt rating. I never use Ray-O-Vacs, I search out Duracells. I never let production by the batteries, I pick them up myself so I can be sure that I'm getting the freshest off the rack.
With the testing, it is not uncommon to have a rejection rejection rate of 20%. (I've had as high as 100% rejection- Ray-o-Vacs).
There are a lot of grandparents out there getting ripped off by the hearing aid battery industry.