Fehling's Test meaning for partial formation of a precipitate?
The reason is thus:

For aldoses (like glucose), the copper sulfate in the reagent easily oxidizes it into the carboxylic acid forming copper oxide (a precipitate).

For ketoses (like fructose), the ketone cannot be oxidized, so there is no color change. Over time at elevated temperature, the α-hydroxy ketone isomerizes to a α-hydroxy aldehyde. This aldehyde is then oxidized to the carboxylic acid, producing a red precipitate.