Our expert says:
Your cats are obviously unsettled for some reason. Perhaps there's a particularly dominant marauding cat around which they don't want to challenge. The only way to solve this is to start all over. Keep them inside for a few weeks with sandboxes, which you need to clean daily. If other cats come into your apartment then you need to stop them getting in and yours getting out. Then you can try moving a sandbox onto the balcony and giving your cats access to it. Perhaps only do this when you are around so you can chase off any visiting cats - use a water pistol or clap your hands loudly. At night keep them inside and other cats out. After a few days move all the sandboxes to the balcony and see if they are still using the inside - I hope not. Perhaps have all the floors professionally cleaned to remove cat odours as these may encourage your cats to go inside. Once they are using the balcony then I'm sure they will use the garden gradually in their own time, but I suggest you leave a box on the balcony for a few months at least.
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