C.B.
i can tell you what we do. my son is 3 and NEEDS his naps. so. we get up at 6 or 6:15, to go to daycare. nap is 12:30 till 2:30 or whenever he wakes up. bedtime is always by 8 pm. this is almost exactly the 12 hours recommended. and yes, i can tell too when he skips. happily that is not that often. but it does happen.
you mentioned you do your same routine every day, but then you mention about ten different tactics you've tried. that's not consistency. it's likely he's in there waiting in anticipation for that hour or two, to see what mommy's going to try next. maybe she'll let me have toys soon! maybe she'll come in and cuddle with me soon! so, i would not keep talking to him/cuddling with him during nap time. i would not allow him to play. i would insist on him being IN his bed with no toys. and i would insist on quiet. mostly this takes care of it for my son. if i really want a nap myself and want to ensure that he sleeps so that i can, i take him to the park or another physical activity, so i know he's good and worn out. do this a few days in a row (heavy on the physical activity in the morning) and if you stick to the same routine, i bet he'll take more naps.
the other tactic i use (when i don't feel he's going to nap, and i'm just wiped out and need it myself), i will let him come into my bed with me and we will watch a movie. correction - he will watch a movie, i will SLEEP. and he is really good about being quiet and laying still so that i can. it's awesome! but i only use this once in awhile, i don't want him depending on that or expecting it all the time. this way it also makes it very special for us both, and we both love the cuddle/quiet bonding time.
good luck!