Venting works in some cases.I do have to agree with JB,on outside
pressure(or in some cases,mold shrinkage or just too thin of a master
pattern).This is particularly true with flat pieces.A little trick you may
want to try is cutting a piece of index card the size of the mold,cut out
a rectanglar piece in the center,leaving a 8 or 10mm border(or a
circle,etc. in a larger size and general location of the piece).Place it
in between the mold plate and the mold,and inject.This will allow the mold
to expand, just a little more.Give it a try.Hope this is of help.