"A knife is missing."
Translation:Manca un coltello.
The verb manca takes the same sentence structure/word order as piacere. (It was being discussed on another question.) Here's a copy of the link someone else posted there that will explain: http://italian.about.com/od/verbs/a/italian-verb-piacere.htm
Gli manca un coltello.
for/to him, is missing, a knife
(Same sentence structure/word order as piacere.)
It's just the way it's done in Italian, I guess. It appears with "sta mancando" you are applying the helping verb logic of English, "to be" verb + gerund; My understanding of this helping verb concept is that it is fairly idiomatic to and much more often used in English than in other languages (and that when used in other languages it sometimes has a somewhat different meaning that what you might intend).
(I'm a native American English speaker and not a native Italian speaker myself, so I don't have any deeper insight to the Italian part of it.)
I think "mancare" is generally better thought of as "lacking." It's less idiomatic in English but it makes more sense. Manca un coltello= a knife is lacking. le manca= he/she is lacking, etc.