When not used for long periods, the batteries should be disconnected inside the decoy to prevent full discharging, because if the battery pack voltage falls below 1.5 V, the charger IC board will not permit charging. However, here are some steps to “resuscitate” a dead battery pack. Warning: you will need a 12 V power supply with alligator clips and a screwdriver.
Ideally you should have a power supply like this, set to 12 V output and current limited to 0.5 A.
Remove the lid of the decoy. Make sure you keep track of the six screws and six plastic sleeves for the screws, which tend to fall out when the lid is upside down. Find a box so that the lid can be flipped upside down while remaining connected to the charging board. Check that all connectors are connected–perhaps that is your problem! In particular, the flat 8-wire connector needs to be together for the charger to work.
When storing for long periods, disconnect the two connectors next to the screwdriver (both white and black/red connectors).