Full Idea
The reason sense-data were immune from doubt was because they were so primitive; they were unstructured and below the level of conceptualisation. Once they were given structure and conceptualised, they were no longer safe from sceptical challenge.
Clarification
'Sense-data' are raw unprocessed experiences
Gist of Idea
Sense-data are only safe from scepticism if they are primitive and unconceptualised
Source
Paul O'Grady (Relativism [2002], Ch.4)
Book Reference
O'Grady,Paul: 'Relativism' [Acumen 2002], p.93
A Reaction
The question of whether sense-data are conceptualised doesn't have to be all-or-nothing. As concepts creep in, so does scepticism, but so what? Sensible philosophers live with scepticism, like a mad aunt in the attic.