positive one month then next month negative

Last month I got a smiley face on the clear blue test it was day 16 of my cycle, this month I've tested day 15, 16 & 17 and all been negative, according to my cycle today should be my day to ovulate. my cycle ranges from about 28 to 33 days, the last 3 months it's been 30 days... i don't get when my fertile time is then or when I will ovulate? is each cycle different? If i dont get a positive result on the opk does that mean I'm not ovulating? very confused ....