Are you watching the All-22 and determining that he's not getting open? Or are you just reading stats?
Stafford mainly targets Puka and Adams even when others are open. It's the way the offense appears to be set up and what Stafford tends to do anyway. While Tutu does appear to be used quite a bit to clear out defenders (if Scott sticks it may be what he's asked to do, too), I've seen Tutu, TEs, and backs open, but prior to just recently that ball almost always goes to Puka or Adams.
BTW, while I might be mistaken, but Tutu is usually on the field with Adams and Puka. I think I seen packages where Smith, Mumpfield, and Whitt are together. I wonder how many of their catches came at this time.