I should've spoken up sooner when the guy I was training still hadn't progressed from the first training module after three days. There were five training modules to go through at one's own pace, and each module usually took no more than a day. Every time I asked this guy how he was doing, he'd answer "Fine! I just need to go over (blah) a bit more, and then I should be ready to proceed with Module 1." Mystified as to why it was taking him so long, I'd ask "Do you need any help? Any clarification?", and he'd answer cheerfully "No! Everything's fine."
I was pretty new at training other people at that time, so I didn't know what to do. Fortunately, my supervisor asked me on the QT how the guy was doing, and when I said "Not well - he's still stuck on Module 1", she said "That's what I thought." He was moved onto another project the next day.