At this young age the most common reason for fighting sleep is actually overtiredness, so the first thing to check is that she's not staying away for too long before you try putting her down for her first nap.
The next thing is to see what you're doing to try to help her sleep. Newborns are often excellent sleepers but around 4-5 weeks old they start to become more alert and then end up actually needing more help to be able to fall asleep.
Swaddling can help, if you're not doing it already. It can really help to control those little arms and legs whose movements may be keeping her from sleeping.
The last question is whether she might be in discomfort. Does she object to being laid down when it's NOT nap time?