Answer:
1. The specimen perhaps hasn't been stained so no structures can be seen.
2. If a light microscope, perhaps the light isn't turned on.
3. The specimen may not bein focus
4. Microscope not turned on
5. Not smeared enough to see the specimen
6. Specimen is too thick - diffraction barrier of light microscopes in which light can't focus the specimen - Low resolving power of light microscope.