This is the same situation that has been taking place (on and off) in our house. My son is 27 months, sleeps very well at night, and used to nap very predictably for 2-3 hours a day. He also talks, sings, rolls, jumps, in his crib (we are on the verge of switching to a toddler bed and think that this may be part of his need for a change) and sometimes does not fall asleep. Lately, I have been trying the following and it seems to have helped: during lunch, I play soft quiet music and start talking in a very calm, quiet voice. Then, after lunch we go straight upstairs to get ready for a nap. I continue using a calm voice and try to encourage him to use a quiet, calm voice as well. We avoid any silly play, but we do read a few books and say goodnight to his toys, books, and a picture of his Dad (who is at work). Then, right before I put him down, I talk to him a little about how to help himself go to sleep: "close your eyes, take a deep breath, relax, think about people and things you love (sometimes we make a list), and think about things you want to dream about." Often, he will get rowdy and silly as soon as I leave. If it continues, I often go back in after a while, remind him that it's time to sleep, sing him another song, and lay him down again. For the last week or so, that has helped him calm down and go to sleep, but I'm not counting on it lasting.
He gets a lot of stimulation and exercise in the mornings at daycare, so is usually quite tired. I find that getting him to bed soon after we get home really helps. If I wait too long, he passes through his tired-time and seems to wake up!
It also seems that lately any change in routine can disrupt naptime. His grandparents were here recently and he didn't nap while they were here, even though he was very tired. We realize that naptime might be on its way out, but are trying to maintain an afternoon "quiet time." Once he is in a toddler bed, we will probably change so that he can be in his room reading or playing quietly, even if he does not sleep. I feel the same sense of frustration . . . especially b/c I am pregnant and I need a nap most days, too!
Good luck!