I have this problem too. My kids dont transition to two naps till 18 months or later and I have about 50% of the kids over the past 6 years sleep a lot at my house and need it desperately. I have a 3 year old right now that will sleep easily 4 hours when she is here. My biggest issue with parents that I have worked with, even the best of parents, is that they have no routine for sleeping. Lots of kids up late, not napping on weekends, getting up early, getting up at night and wandering around their room or even the house
so yeah, its pretty common in my experience. Either the parents just dont know what to do or they are too afraid to say no in word and action. i have had several parents that tell me whenever baby gets up is when the family gets up. If they want to get up and play at 2AM, the parents get up and play with them for a couple hours. Most of my families drop naps at home before 2 years old, even before 1 year old!