Here are the most recent stats.

Combined (Oestrogen/progesterone) pill - Failure rate Less than 1% if used properly.
Progesterone only pill (Minipill) - Failure rate About 2%.
Depot Injection - Failure rate About 2%.
Hormone Implants - Failure rate About 2%.
Intrauterine contraceptive device (IUCD) - Failure rate With the newer coils the failure rate is about 1 %.
Hormone coils - Less than 1%.
Contraceptive sponge - Failure rate About 3-5%.
Contraceptive ring - Failure rate About 2%.
Diaphragm with or without spermicide - Failure rate About 3%.
Cervical cap - Failure rate About 3%.
Sheath male and female - Failure rate 3%-4% in motivated users.

As mentioned above, absorption of the oral contraceptive is reduced by an illness, causing vomiting or diarrhoea - you may have thrown up your pill or it may not have been absorbed. Antibiotics may interfere with the absorption and metabolism of the pill. Once ovulation has commenced it cannot be stopped by the rest of the pills.