15 year olds period

So my 15 year olds period is never regular. She started at age 11 and it's always either slightly early or mostly late by a week or so. I've told her in the past she has to keep up with it and she did good for a while but lately has gotten slack (4 females with periods in the house and I can barely keep up with my own). She tells me now she has no idea when she last had it but thinks it was the 1st of January when they returned to school from winter break. That means she is going on 2 months late. Shouldn't it be regulating by now and if not what is causing this?