Effective ways of encouraging "weak" students to make use of office hours
I have found personal (email) invitations to be very effective at getting certain students to come to office hours. (After attending once by personal invitation, they seem to be much more likely to come again without an invitation.)

The emails typically look something like this:


  Hi Jane,
  
  I noticed that some students, including you, had trouble with the theoretical basket weaving questions on Homework 3. I would be happy to review this topic with you during my office hours this Monday, 3-6PM in room 901. If you have class or other obligations during that time, you can email me to set up an appointment at another time.