Not enough in the midfield, you have played a formation that gets the possession penalty, we call it "Illegal" formation here on the Forum.

From http://wiki.topeleven.com/Squad:
"Some formations are not allowed in Top Eleven and may lead to 20% ball possession, which in turn reduces the chance of victory.
Throughout the whole match you must have:
a goalkeeper (blue)
at least three defenders (green) (DL/DC/DR)
at least one striker (red)
at least one player on both flanks.
at least four players in the opponents half (ML/MC/MR or AML/AMC/AMR or ST)
at least three players in the midfield section (AML/AMC/AMR and/or ML/MC/MR and/or DML/DMC/DMR)
One player on each flank is enough, but three players on the same flank will also lead to 20% possession."