Just a question :)

I just have a quick question. I am a bit new to this app and trying to conceive. According to this app I am in my fertile week however I am also using the opk clear blue digital and I have been doing this for the last 6 days. I just keep getting a blank circle and my fertile days keep getting pushed back on the calendar. I am getting a bit discouraged. Just wondering if this is completely normal and I probably just ovulate later in my cycle or if I should start to worry alittle? Thanks for any and all input!

:)