I know from experience I've had a good number of emails go to my "spam" folder even though they are not spam.
Some commissioners also get A LOT of emails and honestly don't have time to respond to every one of them. From my own experience, there have been times where I would spend 3-5 hours JUST doing email (send quotes, regrets, updates...) and it's on top of a full time job, commission sewing, my own sewing, family/life, and school. Every day was at least 1 hour of emails and then the 3-5 would be on, let's say, a Sunday night.
It's really hard to keep up on. Once you fall behind, it's a constant catch-up that never ends. It's just one person, doing everything, from work to customer service. Sometimes, you just gotta let some slide and unfortunately not respond.
Excuse me, Please and Thank You will get you far in life.