A staff change and phrasing slur problem

[Robert Blackstone]

Dear all,

I have been struggling for many hours with a pianoscore in which the mostly 
polyphonic text regularly switches between the treble staff and the bass staff.
There should be (phrasing-) slurs joining the beamed groups of notes.
Sometimes these slurs do not appear, as in the upper voice  between b.2 an b.3 
in the ME.
Probably I do something wrong but I have not been able to find out what.

Can someone help me out?

Thanks in advance.

Best regards,

Robert Blackstone


%%%%%%%%%%%%%%%%% 
\version "2.18.2"

upper  = {
  \clef treble
  \time 3/8
s8 s8 s8 | %1
s8 s8 s8 | %2
s8 s8 s8  | %3  
 }

lower = {
  \clef bass
  \time 3/8
  << { a8 [ b ]  \( c' [  \)  }  \\ {\stemDown  c8 [ d ]\( e [ \)  } >> | %1
<< { d'8 ] \( \change Staff = "upper" e' \) [ f' ] \(  }  \\ { \stemDown f8 \(  
] g \)  [ a  ]  \(   } >> | %2
<< { \change Staff = "upper"  g' \)  [ a' b' ]   }  \\ { \change Staff = 
"upper" \stemDown b \)  [ c' d'  ]  } >> | %3  
}

\score {
  \new PianoStaff 
  }
  <<
    \new Staff = "upper" \upper
    \new Staff = "lower"  \lower
  >>
  \layout { }
}
%%%%%%%%%%%%%%%%%%%%