ADG719 strange behaviour


I am using the ADG719 switch. I have two inputs with a frequency of 125kHz. I am switching between the two using a logic signal of 250kHz. But when I look at the output of the switch I just see a pulse of one of the signals then about a gap of 12ms then a pulse again. Its as though the switch is only letting through one of the signals every so often. Anyone have any idea why this would be?