I once had a similar problem. The crystals only reproduced when grown in condition containing PEG 8000 from SIGMA but not from FLUKA. This difference may not affect crystals from other proteins but some proteins are more sensitive to slight changes than others. Since you found out that the stock is not available, it will be worthwhile to try making stock containing PEG 4000 and maybe also try PEG 4000 from other suppliers.
-Raj
