I have a SUMPRODUCT formula that I know is OK but returns the answer 00:00:00

=SUMPRODUCT(--(Sheet2!B4:B573=A6),(Sheet2!C4:C573))

Sheet 2 is full of cells with times (HH:MM:SS) which have been pasted in
from another excel worksheet.

However, if I go into each cell in sheet 2 and return, the formula then
works €“ its as though the cell value is hidden.

Hope you can help?