SO CONFUSED...OPK TEST

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So i am cd 7, noticed small amount of ewcm when i wiped so thought id try an opk. I do have irregular af following stopping bc in april. So my opk strip by one step clearly shows a surge but my dual hormone clear blue opk says no..not even flashing. Same urine at 7.50pm. last drink at 1 pm so more than 4 hours so time scales are right. HOW CAN IT BE SO DIFFERENT?????