Hi. I Have Had A Neck And Head Injury From

Hi. I have had a neck and head injury from a car accident nearly two years ago and I get very bad headaches. The doctors gave me sevredole 10mg for when it was very bad. I have been managing my headaches with panadole/paracetamol but last night I had a very bad headache and I took one sevredole then waited an hour and had to take another.... This was unusual as I don't usually need to take two but even taking two didn't help and I've had an awful sleep and have woken up with the same bad headache. Why has the sevredole not worked??