Unpopular nest boxes

With all the questions raised by Jessica's research, I have decided to keep conducting winter field seasons. Monitoring has started Nov. 2014 with Michael Gates. Despite a fairly cold winter so far, only 3 downy woodpeckers, 1 tufted titmouse (first in 3.5 winters), and 4 female bluebirds (only 1 individual previsouly banded as a chick June 2014).

Why are the nest boxes so unpopular with roosting birds this year? too much disturbance caused to boxes by our visits in previous years? not freezing enough? not late enough in the season? or because this year, I have not provided nests in any of the boxes?