Avoid break page between two subsections and strange behavior

I am working with a template from a book and this is the problem is the following:

I have two sub-sections, each with an itemize, but LaTeX puts them on different pages, but clearly, there is enough space for the two subsections to be put on the same page.

Everything becomes weirder when I include a letter between the subsections because although there is more text there, they are put on a single page. Which has me very confused.

Could you help me, please?

Greetings,

subsubsection*{Definición de Conjuntos}

begin{itemize}
    item $I:$ alternativas de inversión ${1, 2, 3}$.
    item $J:$ personas ${1, 2, 3, 4}$. 
end{itemize}

subsubsection*{Definición de Parámetros}

begin{itemize}
    item $C_{i}:$ valor inversión en artefacto $iin I$.
    item $PV_{i}:$ precio de venta de alternativa de inversión $iin I$.
end{itemize}

An image of the problem:

The result when I add more text:

Edit:
Thanks for your answers, I attach the .cls file and the general configuration (preambulo.tex) that I am using.