Hi!
If you were my patient, I would first assess whether you are having regular periods/cycles. To do this, ovulation predictors can be useful. I would also advise a sperm count for your partner (even if he is the father of the other kids, things can change). Spermcheck is available on line if he doesn't want to go to the doctor. Finally, if both of those parameters are in order, I would suggest a saline ultrasound or hsg to check the uterus and fallopian tubes to be sure there are not problems there. If you DO have irregular periods, you may need some blood work done to evaluate why.
Good luck!
Dr B
To add to the matter I have this period tracker on my phone its been acurate for me every month but this month I was due to start my period on the 3rd and it says I'm 3days late