There are many possible reasons why words are dropped.
Word is a "stop word" (is, are, etc)
Words got "stemmed" (fly = flies)
Shape is too complicated
Not enough words (so some words become too big to fit)
Font is tricky
In general fitting words to shape exactly is a very hard problem.
The only way to guarantee all words are used is to use "classic
shape" (aka shapeless) or to reduce the tightness (but words will not
fill the shape).