Hi Sherif, successful crystallization depends on a lot of different aspects, and it is difficult to predict what effects optimization trials will have. But at least you should make sure the protein is very pure, as well as monodisperse (to be assayed e.g. by dynamic light scattering) and stable.

- To optimize crystallization conditions you may vary concentration of the different components, pH, try different buffers, use additives.

- What is the size of the current crystals? Could bigger crystals be grown?

- Did you check if the cryoprotectant is negatively affecting crystal quality? For example if possible you can do a room-temperature diffraction experiment without glycerol to see what is the 'native' diffraction quality.

- If still unsuccessful you may want to change the construct by slightly varying the length of it, or to remove the tag (if you have it).

Hope this helps, Tjaard