Pigeons quite laying

I have 6 pair of birds tat have all had one pair of young this spring. 3 pair still have young in the nest boxes but they are about ready to leave and the other 3 pairs young are out of the boxes. None of the pair have laied a second set of eggs yet, in the past they would usually have the second set laid before the first set had left the boxes. My boxes have two halves with two nset bowls, so there is plenty of room. I am feeding a good quality high protien feed and am supplying grit. All the birds look and act very healthy. Any ideas on what might be wrong?