systems overlapping

[Rodolfo W . Zitellini]

Hi,
I'm quite new to this program, and for exercise I was trying to typeset 
the score of a mass, with nine staves.
I have made a separate input file for each voice and I include them 
using \include in a top level file containing the \score context. While 
the music gets typeset correctly, I get that in every page there are 
two systems that overlap. I have read trough the manual but I didn't 
find any help: I tried changing the font size (but I always get two or 
more systems overlapping!), and forcing page breaks.
Can anyone help me?
I am using lilypond 2.3.1, and my top-level file looks like this:



\header {
         title = "Messa a 4 La Lambertina"
         composer = "Giacomo Antonio Perti, 1736"
       }

\include "violino1.ly"
\include "violino2.ly"
\include "viola.ly"
\include "violoncello.ly"
\include "canto.ly"
\include "alto.ly"
\include "tenor.ly"
\include "basso.ly"
\include "organo.ly"

#(set-global-staff-size 20)


\score { <<

\context StaffGroup = "orchestra" <<
        \staffA
        \staffB
        \staffC
        \staffD
                >>
        
\context ChoirStaff = "coro" <<
        \staffE
        \staffF
        \staffG
        \staffH
                >>

\context StaffGroup = "basso" <<
        \transpose c,, c, \staffI >>

>> }

thanks,
Rodolfo