Hello Alper,
Probably some of the original images where not aligned properly, or the selection of the image for the blurred areas was incorrect. You can try to draw large polygonal patch over orthomosaic and look through the suggested images in the Assign Image dialog, to figure out, which image is blurry - then review the image and check, if it is aligned properly. Or just exclude such images from the orthomosaic generation process, if there are only few.