Hi everyone! My baby is 3 weeks old. There's milk, she wants to nurse …

Hi everyone! My baby is 3 weeks old. There's milk, she wants to nurse and I offer the breast, but she barely sucks and then gets upset, shaking her head like she can't latch on. Has anyone experienced this? What's the cause? How to deal with it? It happens during the day — at night everything is fine, it only starts closer to morning and then lasts the whole day.