Different ovulation tests


I took 2 ovulation tests in the morning (cheap brand and advance clearblue)

Cheap= +

Advance clearblue= blank circle

Then I took 2 more in the afternoon.

Cheap= —

Advance clearblue= black circle blank

Why did I get a positive with cheap and not with advance clearblue in the morning? And why did the cheap go from + to — in a few hrs?