Good advice here. A few other thoughts... Does her preschool have a set up where you can go and observe without her knowing you are there? That way you might be able to see some things that your daughter or the teacher could do differently.
How dark is the room? In my own personal opinion, it should be dimmed but not dark. Some centers no longer darken a room at all because of fears of accusations of improper touching during naptime (by teachers or other children). I like a room where the light is subdued to allow for better sleeping, yet light enough that anyone coming in can easily see all the people in there and would be able to detect any impropriety that might be occuring if there was such going on.
Most children of this age don't like to settle down, and would rather be going full blast all day long. They are also at an age where they are trying to exert their own independence, and most of them seem to think they are 'all growed up' and can do anything they want on their own ("I don't need an adult telling me what to do, 'cause I can do it by myself"). This is a good stage for the development of the child, but difficult for us as adults to cope with. My own granddaughter is currently sleeping at the end of my hall on the floor, because she was going through the don't want to sleep routing today. When she kept being disruptive to the other four children in my care, I moved her cot into the hall so she could play without disturbing them. She decided she'd show me by sleeping on the floor... and that's where she is at the moment. It won't hurt her to sleep on the clean carpet, and she is getting the sleep I knew she needed, but she wouldn't admit to needing. I'm sure your daughter's preschool teacher doesn't have the freedom to allow that drastic a solution, but she might have a place to move your daughter where she could be more active on her cot without disturbing the other children. Sometimes there is a shelf unit dividing a room and one child could be located on a side of it away from most of the other children, yet where the teacher can easily observe her, and given books or soft toys that she can play with on the cot.
Does your daughter have a regular nap routine at home? If so, is it at approximately the same time as the one at daycare? This is sometimes a help to the child because it's difficult to have one routine at daycare and a very different one on the days she is at home.
I do agree with the person who mentioned she might not have had enough activity before naptime. We try to get our kids outside in the morning for a lot of hard running and playing. Just before bringing them in for lunch we have them race to the fence and back three or four times just to be sure they've run hard and tired themselves out. Centers often have to have classes share playgrounds, and have limited time frames... and of course some classes will have their outdoor times earlier in the morning in these cases. If that's a problem, perhaps you can help suggest some ideas for the teacher to do some really active indoor activities, like some jazzy music to get the kids dancing around the room, or an appropriate active indoor game. If the program is concentrating on more of the "seatwork" or 'paperwork' type of activities, you might want to suggest that they consider adding more active games... most children learn better through those anyway if they are used in conjunction with the concepts the school is trying to teach.