Here is my suggestion for your case.

1. To verify the crystals are protein, try to fish all of crystal cluster out on a small loop, and then have a exposure on X-ray. Based on the diffraction pattern, you can tell it is protein crystal or not. 

If it is protein crystals, then: 

2. At this stage, try different buffer with the same pH. Different buffer (different ion strength?) can help you get crystal with different shape or morphology. When I worked for my client, they had their crystal always crystallized as very thin sheets (Image_1.jpg). After I changed the buffer, it turned to a long and reasonable thick bar  with much better diffractive quality and radiation stability (image_2.jpg). 

3. Please also try this: add 5% 2-propanol/ + 5mM (NH4)2SO4 for charge reshuffle. 

4. try some precipitant other than PEG(PEG8K, Image_3). After I change PEG8K  to Poly(acrylic acid sodium salt 5100), the crystal became even visible in 96-well (Image4_1).  The reason to select Poly(acrylic acid sodium salt 5100) is its pH range (6.3 ~ 7.1). 

However, each crystallization is unique. I hope my suggestion is helpful.

See your publication soon.

Best

Kevin