Coloured cells in Tabular printing over table lines

Question

I'm using Tabular and XColor to color some cells of a table, but some of the coloured cells are "printed above" the lines of the table (see attached image, which shows part of the table). How could I avoid this?

Thanks in advance for your time and effort! :)

\begin{center}
\begin{tabular}{|l|cccc|cccc|cccc|}
\hline

(...first part of the table, not relevant...)

First item & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & & & & & & \\
\hline
Second item & & & & & \cellcolor{gray!25} & \cellcolor{gray!25} & & & & & & \\
\hline
Third item & & & & & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & \cellcolor{gray!25} & & & & \\
\hline

(...the rest of the table...)

\end{tabular}
\end{center}