Does it fit properly? If it rides up their face and covers the eyes it will obviously upset them. They won't be able to see ;) :) Mine use them, but it did take a while to get used to them. Little and often, feeding them special treats helped :D
Our first attempt - i.e. put it on, try and take her for a walk - was a unmitigated disaster. :rolleyes: and was following advice from someone I'd not listen to again.
I then read on here to introduce it really slowly, so a left it a month or so, and got a different one altogether and started by showing it to her and giving her a treat. I did that for a couple of days. Then I started having a treat the other side of the nose loop so she'd put her nose through it. Did that for a few days. Then slipped it onto her nose and took my hand away....for a few days. Worked up to to her wearing it for meals...anyway you get my point, tiny steps forward.
We don't use the head collar so much now, but if we do she'll sit really well in front of us and poke her nose through the loop to put it on.