purifying phosphates on sugars

By thin layer chromatography, I can observe a spot with low rf that
should correspond to acetate-protected mannose with a diphenylphosphate
at the anomeric position; however, when I attempt to purify the product
via column chromatography I never observe the desired material. I
suspect it is the result of the phosphate being hydrolyzed. My current
column conditions are as follows:

gradient elution: 10% EtOAc, Hexane to 100% EtOAc using silica gel

I have tried pretreating the silica gel with TEA, and I run the column
as quickly as possible.

any other suggestions, concerns, comments would be more than welcome.
Thanks.

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