It is normal for a regular period to become weaker or even completely stop when start taking contraceptive pills including mini pills. Mini pills contain only progesterone. They are taken every day without pause and there aren't any inactive pills in the pack.
In 50% of cases these mini pills can prevent ovulation and those women won't have any regular periods. Those women can establish regular periods after taking off the pills.
In the rest of the cases (the other 50%) ovulation is not prevented and those women have regular periods even if they take the pills every day.

so on the mini pill is there an increased chance of pregnancy? since in 50% of cases ovulation is not prevented?

Mini pills (progesterone only) act like a contraception method not by preventing ovulation but by making the cervical mucus more viscous that prevents spermatozoids to move toward the uterus. Combined pills (estrogen plus progesterone) acts by both mechanisms - preventing the ovulation and thickening the cervical mucus and that's why they are more efficient than the mini pills. But still, mini pills are efficient enough if used properly.