The rut is a whole cycle. Lockdown is the part of the cycle where the does are actually bred. When this happens, a buck and doe pair off, often for 24 hours. They hang out and do the deed, usually in a secluded location. Romantic! Those pairs are MIA because they aren't moving around much and aren't on a regular pattern. They're holed up in a thick spot, out of the way spot, oddball spot.
Hope that explains it a little. Maybe someone else can chime in. The basic idea is that they are busy gettin' busy, so they don't walk by your blind/treestand.