Count, moving diamonds, failed to impress (3,2,3)
I believe the answer is:
cut no ice
'failed to impress' is the definition.
(to cut no ice is to fail to impress someone)
'count moving diamonds' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'diamonds' becomes 'ice' (slang term for diamonds).
'count' with letters rearranged gives 'cutno'.
'cutno'+'ice'='CUT NO ICE'
(Other definitions for cut no ice that I've seen before include "Not make any impression" , "Fail to split frozen water and count or nothing" , "Be ineffective" , "Blunt skates will" , "Fail to impress" .)