1. Drop contains non-protein particles.
If the coverslides were not clean, you can find pieces of glass in the drop. Glass has an irregular shape without birefringence.
Fibers from clothes are colorful under polarized light.
Dust on the cover slips, in and around the drop: clean the cover slips carefully with a tissue before setting up the drops.
Dust in the drop, but also in the well: filter the stock solutions.
Bacterial growth: add sodium azide to the protein and well solution.