During seeking and chasing, grunt sequences and short rattling sessions imitate the buck-on-buck competition that mature bucks investigate. During lockdown, the breeding buck has nothing to gain by responding — he already has what every other buck is competing for.
If you suspect lockdown, shift to passive setups near suspected bedding pairs and trust the wind. The day after lockdown — when the breeding buck loses interest in his cycled-out doe — calling effectiveness comes right back. See state peak-rut dates to identify the lockdown window for your area.