The most common cause of poor nuclear detail lies in the processing of the tissue; the fixation, dehydration, or clearing step was not adequate. The processing schedule should be reviewed and the times and the number of changes in each of these steps adjusted to ensure sufficient fixation, dehydration, and clearing of the tissue.
Another less common cause could be that the tissues or slides were exposed to excess heat somewhere during the processing or drying procedure. The temperature of the paraffin baths on the processor should be monitored daily to ensure that proper temperatures are maintained.
The temperature of the slide drying oven should be maintained just a few degrees above the melting point of the paraffin being used.
Compare the lack of nuclear detail in the top image with the crisp nuclear detail shown in the bottom image.