Not unheard of see section 5. [1]

It could be because the concentration of the either the protein or precipitate is too low so you should increase either on a gradient to make a closer path to the nucleation zone. I have tried that for the protein I study and it just results in irreversible amorphous protein precipitation. The slower week to month long path is better for the protein I work with. Over time the concentration of both protein and precipitate will increase as the droplet evaporates. Then the protein will either come out of solution as precipitate or crystal or both.

Maybe you have some antinucleating agent like glycerol in your original protein that is slowing down crystallization or maybe the optimal pH is off by 0.5 or more. Optimal pH for enzymatic activity =/= optimum crystallization pH.

One thing you could do is change the temperature of crystallization tray to 4C (cold room), 18C, or 25C. It is a little risky, though, because un-crystallization can happen. Temperature controls the rate of protein crystallization but higher temperature =/= faster crystallization. For some proteins, crystallization is faster and only really possible at 4C.

Source:

[1] Luft JR, Wolfley JR, Snell EH. What's in a drop? Correlating observations and outcomes to guide macromolecular crystallization experiments. Cryst Growth Des. 2011 Mar 2;11(3):651-663. doi: 10.1021/cg1013945. PMID: 21643490; PMCID: PMC3106348.

Article What's in a Drop? Correlating Observations and Outcomes to G...

Thanks Adrong, but I am actually looking to see if anyone knows of a commonly crystallized protein that exhibits slow crystallization growth!

In the few such examples I am familiar with, such very slow crystallisation was due to some chemical changes happening to enable crystallisation. E.g in

https://www.sciencedirect.com/science/article/abs/pii/S0022283601946637

a scFv selective for the non-hydrolysed ampicillin crystallised quite fast in absence of the hapten. In presence of the hapten, crystallisation was delayed by about two months until spontaneous hydrolysation of the beta-lactam bond ring reduced the concentration of the intact hapten to a level that allowed the scFv to crystallise in its apo (unliganded) form. Personal communication with protein crystallographers indicated other examples where e.g. the cleavage of a flexible tail by a low level protease contamination produced a form of a protein capable of crystallisation.

Protein crystal growth affected by temperature, pH, protein solution stability, saturation, nucleation, impurities and so on. Decreasing the temperature during nucleation works to slow down the crystal growth. And one should try above mentioned parameters.