Thank you for the detailed follow-up — since ARC sync works correctly on your Android phones on the same Wi-Fi, we’d like to narrow this down to the specific Fire OS device.
Could you please confirm a few details about the tablet?
Exact Fire HD model (e.g. Fire HD 8 10th / 12th generation)
Total RAM installed on the device
Available free storage at the time of the sync attempt
ARC’s initial sync can be memory-intensive, and Fire OS devices vary quite a bit in available RAM and background process limits, so this information will help us determine whether the behavior is related to device constraints.
Once we have those details, we can advise on the next steps.