i never had a problem with my pill. been on them since last year. like the other member said, could be stress or just one of those months. if she doesn't get a period 2 months in a row on bc pills, then its a concern.
Certain types of medications can delay periods or counter act with Birth Control pills. Especailly anti-biotics, it can sometimes cause BC pills to not be effective.
Stress can cause missed periods. Has she maybe been stressed more often lately?