Hi,
Your textures must be expressed as powers of 2 but values like 1024x256 or 2048x512 for example are also accepted.
On the other hand, it is possible that the materials in Blender are corrupted (various manipulations can lead to this, such as multiple texture changes for a given material). In this case, the textures are not exported.
I had this very often with *_comp.png textures. In this case, I just delete the material, go through the "Orphean data" menu in the Outliner window to remove it from the current model and save. Then I reopen the model, recreate the material correctly and it works fine