I took CCNA class before, and one of the instructors just don't know how to teach.
They exist, I worked with one... I had his former students that couldn't subnet, so we had remedial subnetting exercises till they got it.
They were easy to sort out. After calling roll on the very first day, I gave them an exercise to design, build, and configure a network to ping from PC to PC across 3 routers in series, and they were only allowed to use addresses in the 192.168.1.0/24 network. They could use static or dynamic routing.
topology
(PC)--------(RTR)---------(RTR)---------(RTR)---------(PC)
My previous students typically knocked it out in 20 minutes. His previous students, not so much, most couldn't do it. But I got them caught up by doing subnetting drills to start every class for a couple of weeks.