If you’re using a citation style from the harvard package in LaTeX, you might be surprised to find that
\cite{Kremer2012, Campbell1999}
only works on the first citation
(Kremer et al. 2012, ?) .
Unlike some citation styles/packages (notably the default ‘plain’), the harvard package doesn’t allow spaces between citations. To fix this problem just change the example above to
\cite{Kremer2012,Campbell1999}
and your problem is solved!