I have a 20 month old who had a similar problem. At the age of about 3 months, she suddenly decided that afternoon naps were not necessary. I would put her down, and she fell asleep really well, then woke after only 20 min and wouldn't go back to sleep. I'd rock her, and occcassionaly get her to sleep a bit more, but generally not, and then she'd be really cranky!! I'd sometimes try for over 2 hours, just like you.

We just worked it through. Every day we tried, and sometimes we succeeded. She would be exhausted, but sometimes just wouldn't sleep. The doctor put it best when he said "you can't make them sleep". Grace started daycare at 4 months when I went back to work, and it was no better. Eventually though, she did finally find her routine. Now, at 20 months, she sleeps 1 1/2 to 2 hours every day - no matter where she is. One thing that helped, a VERY dark room. We keep towels, blinds, and curtains over her window. She likes to be very warm. Also, when she was little, daycare put her to sleep on her tummy, and she slept on her back - and couldn't roll over.

We tried tons of external stuff (light, warmth etc.) before we found the right combination - a dark, warm room, and the same music playing every single time we put her to sleep (nap time and night time). It works like a charm.