I am not sure it was from lack of trying as all their efforts ended very tragically.
Anne's final pregnancy ended on 25 January 1700 with a stillbirth. She had been pregnant at least 17 times over as many years, and had miscarried or given birth to stillborn children at least 12 times. Of her five live born children, four died before the age of two, the cause being reported as chicken pox.
There were also other reports her hubby had suffered an injury and he had damaged sperm contributing to the infant mortality results, I am not sure how this could be determined in the 1700's !