As some have mentioned, it could be a function of whether you're wearing regularly or not. My thoughts are, after being 24/7 for nearly three years, is that you can try to plant the idea in your head, as you fall off to sleep, but you are rarely rewarded with a diaper dream from that. If you're wearing regularly, then there's more of a likelihood of having a dream where you're diapered, but you don't even sense that in the dream. The mind is a strange creature. Supposedly, we have a dream every 90 minutes, in REM sleep. If you train yourself to wake up, and have note paper and a pencil handy, you can jot a few notes about a dream you just had, and you'll usually remember it in the morning, once you reread the notes. I can remember (recurrent) dreams I had as a kid, even when I wasn't writing anything down. They just happened often enough, and I probably woke up enough times in a cold sweat, that I started moving that dream from my subconscious to my conscious state. Dreams are usually created by your brain, from very recent (last few days) activities you've had, and the brain attempts to stitch them together into some sort of bizarre story, in effect, maybe to keep you involved enough to keep you sleeping, so you can get your rest. Those with c-pap machines probably don't see much REM, so perhaps they might have more trouble having or remembering dreams. But, there are no hard and fast rules. Sometimes the dream is about people you haven't thought about in years, but again, that's the brain just trying to create a story.
OK, that being said, my last diaper dream was me at a carnival of sorts, when three guys/gals came up and asked if I was wearing a diaper. I said no, and they grabbed me, threw me to the ground, ripped off my pants & underwear, and proceeded to diaper me right there, and then ran off with my clothes. I was deeply humiliated, almost crying. My wife (in the dream) was somewhere nearby, and when she found me, wandering in a diaper & shirt, she made me continue to wear that as we made our way to the car. I was upset, but I understood why she was doing it. The dream trailed off to us walking out of the venue, with my humiliation there, for all to see. I woke up, in a sort of huff, and remembered the dream contents, and moved it to my conscious mind, without having to write it down, and I still get that weird sense of humiliation at times, when thinking of the dream. I've had others, but that was the most recent one that I remembered. Others occur where I know I am wearing, and going about whatever the dream is about, but somehow, I don't manage to bring the diaper aspect to the forefront, and I end up not remembering the dream. The brain's subconscious activity really is a strange thing. It's sort of sad that some people don't seem to remember any dreams, which may be a matter of how they wake up, and move into their conscious life. Try keeping a pad & pencil right there, and telling yourself you want to wake up when you're dreaming. I learned all of what I know by studying it in college. Results may vary, but the big part is understanding the process. My wife goes to sleep sometimes, and in 10-20 minutes, I can tell she's dreaming, so she's the exception to the rule.