I have been trying to capture a handshake of a certain target with airmon-ng, it's a strong signal -50db with three or four clients associated.

I just cant seem to successfully de-auth this site. I've had a little de-auth loop running, see below, for half an hour and nothing. No handshake captured. Same approach works fine on other targets.

while true; do aireplay-ng --deauth 16 -a 00:11:22:33:44:55:66 wlan1mon; sleep 15; done

Why would this one target be so resilient to de-auths?
Is my only option to wait for someone to legitimately log on to get a handshake?