Ovulation tests

Hello me and my partner have been ttc for awhile now and no luck yet. 

Ive been using cheap ovulation tests and they have been coming up positive but if i use clear blue they never come back positive. 

Is it possible the cheap tests are wrong and that im not ovulation or does clear blue just not work properly? 

thankyou for any replies because im confused and my doctor thinks its also strange so any help would be great! x