Ikea Pax wardrobe assembly help

After assembling the Ikea Pax wardrobe, the Pax back panel actually fits quite well. However, the next day the back panel of my Ikea Pax wardrobe warps so much that gaps form between the individual nails, resulting in unsightly spaces. How can this be prevented or fixed?

Take a photo.

And, have all the nails for securing the back panel been used?

I always use 2.5 mm screws for that. Every 50 cm apart.

In the areas where gaps have now appeared, hammer in a few nails and also hammer the nails all the way through. Then the unsightly gaps should be gone.
Best regards

I have now taken another photo. I hope it is clearer now what I mean.

Take a photo.

And, were all the nails used to attach the back panel?

Yes, all the nails were used.