\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[full]{textcomp}
\usepackage[greek,italian]{babel}
\usepackage[margin=2cm,top=1cm,headheight=16pt,headsep=0.1in,heightrounded]{geometry}
\usepackage{siunitx}
\sisetup{inter-unit-product = \ensuremath{{}\cdot{}}}
\usepackage{array}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}
\usepackage{caption}
\captionsetup[figure]{justification=centering,labelformat=empty}
\newcommand{\desc}[1]{\caption{\hspace{0.5cm}#1}}
\begin{document}
\begin{table}[htbp]
\centering
\caption{Aggiunte ($\mu$L) della base titolante \textit{n}-butilammina (0.102 M) per i catalizzatori HY e $\text{Al}_2\text{O}_3$.}
\begin{tabular}{|P{5cm}|P{1.5cm}|P{1.5cm}|P{5cm}|P{1.5cm}|P{1.5cm}|}
\hline
\multicolumn{3}{|c|}{\textbf{HY (0.1016 g)}} & \multicolumn{3}{c|}{\textbf{$\text{Al}_2\text{O}_3$ (0.1008 g)}} \\ \hline
Tempo di attesa (min) & $V$ ($\mu$L) & Viraggio & Tempo di attesa (min) & $V$ ($\mu$L) & Viraggio \\ \hline
10 & 250 & no & 10 & 20 & no \\ \hline
5 & 200 & no & 5 & 20 & no \\ \hline
5 & 150 & no & 5 & 10 & no \\ \hline
5 & 150 & no & 5 & 5 & si \\ \hline
5 & 100 & no & 5 & 5 & - \\ \hline
5 & 50 & si & 5 & 5 & - \\ \hline
5 & 25 & - & - & - & - \\ \hline
5 & 10 & - & - & - & - \\ \hline
\multicolumn{3}{|c|}{Volume titolante totale: 900 $\mu$L} & \multicolumn{3}{c|}{Volume titolante totale: 55 $\mu$L} \\ \hline
\multicolumn{3}{|c|}{Moli titolante utilizzate: 0.918 \unit{\milli\mol}} & \multicolumn{3}{c|}{Moli titolante utilizzate: 0.0561 \unit{\milli\mol}} \\ \hline
\end{tabular}
\end{table}
\end{document}
As you can seen, vertical-align text works in some cells and doesn't work in others.
Where is the problem?
