I had that happen a few times but it was months ago. A handful of my 5min scents had nothing for the 1st spawn. It only happened for a month or so, and then i assumed they fixed it. I guess not.
Yeah one morning I did an epic scent at my house and nothing spawned the whole 20 minutes. I was so mad. I contacted support and they gave me another and it worked fine.
Ive had this happen to me in the past… normally force close and a restart fixed it for me… havent had it happen in a while though.
General rule of thumb force closing and restarting your game is the official Ludia fix for any bug or other issue so when in doubt try that.