The longer that you're on birth control pills the more likely it is that you will cease to have a period. It does not mean that the birth control pill is not effective. On the contrary it is working very well. The lining of the womb has become so thin that there is nothing to shed. In other words the progestin has won out over the estrogen
. Many women actually like this as a side effect of taking oral contraceptives. You will have to take a stronger estrogen containing pill to avoid the side effect but I don't think there's any medical reason that you need to.