Clear Blue Ovulation testing Kit

Hi,
I have been using this kit for a while, worked for my first child perfectly. However this month I tested Low, then High Peak and then back low???? does anyone know why or has it happened to them. 
I have followed the instructions on the kit to the T. Im confused....
my cycles are regular and I don't have any issues in that respect. 
Any info would be appreciated. 
Thx

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