The first page of chapter is left blank, while the figure is drawn on the next page?
\chapter{Diagrams}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\tikzset{user/.style={circle, inner sep=0pt, minimum size=0.6cm, fill=Cerulean, draw=none}}
\tikzset{usersAOI/.style={circle, inner sep=0pt, minimum size=5cm, fill=Bittersweet!30, draw=none}}
\tikzset{object/.style={inner sep=0pt, minimum width=0.4cm, minimum height=0.4cm, fill=WildStrawberry, draw=none}}
\begin{figure}
\begin{tikzpicture}
\node[usersAOI] at (5,6) {$A$};
\node[user] at (5,6) {$A$};
\node[user] at (7,6) {};
\node[user] at (6,4.5) {};
\node[object] at (6.5,5.5) {};
\node[object] at (5.2,4.5) {};
\node[user] at (6,1.5) {};
\node[user] at (8,3) {};
\node[user] at (10,3) {};
\node[object] at (9,3) {};
\node[user] at (10,7.5) {};
\node[user] at (12,7.5) {};
\node[user] at (10,6) {};
\node[user] at (12,6) {};
\node[user] at (11,7) {};
\node[object] at (11,6.2) {};
\node[user] at (15.5,6) {};
\node[draw,align=left] at (15.5,5) {zzzzzzzzzzzzzzz};
\node[object] at (15.5,4) {};
\node[draw,align=left] at (15.5,3) {iiiiiiiiiiii};
\draw (15.5,2) [fill=Bittersweet!30] circle (0.3cm);
\node[draw,align=left] at (15.5,1) {zzzzzzzzzzzzzzzzzz};
\end{tikzpicture}
\centering
\caption{uuuuuuuuuuuuuuuuuuuuuu}
\end{figure}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%