Hi,
According to me in case of Asset Shut down no depreciation is posted for that particular asset. So in this case it may be possible that the Asset was shut down during the Useful life of asset and then till the shut down period no period was counted and once the shut down is removed then system will start depreciation again. Also their is configuration in Base method assigned in the Depreciation key which says about Depreciation to be posted below Useful life or not. Please check the above things.
Regards,
Malhar.