Bobbin thread won't pick up and tangles up in a...

Question

It seems to happen when the bobbin is half full. The machine starts making a clicking sound and a stutter.Then the balled up mess underneath. I took it in for cleaning and told them what was happening but obviously they didn't check it. I have rethreaded the top of the machine, have removed the bobbin and put it back in and have put in a new needle. When the bobbin is full it seems to work fine.