I think this is a good reason why -
In 2011 and 2012 at Stithians I had 15 occupied boxes. Due to last year's poor season and a cold Spring, this year I had 10 occupied. 6 boxes of Blue Tit produced 40 eggs, 27 pulli, 24 fledging giving egg to fledging success of 60%. Although 8 boxes were taken last year the averages for eggs, pulli and fledging per box are almost identical. For Great Tit last year 7 boxes were taken, 32 eggs, 5 fledged giving egg-fledging ratio of 15.63% . This year 4 boxes were taken 22 eggs, 9 fledged giving a ratio of 40.9%. Much better despite the lower occupancy.
Down the road from my house I have 2 schemes 200m apart from each other courtesy of different landowners. Same habitat. Both had 4 boxes taken 2 Great Tit's each and 2 Blue Tit's each. Scheme 1 had 22 eggs of which 10 fledged giving a success rate of 45%. Over the road and the four boxes produced 1 egg less at 21 of which 19 fledged giving a fledging rate of 92%. Astonishing difference between the two.