I work as a dispenser in a pharmacy and drug shortages are the bane of my life at work. There are several reasons why drugs become unavailable,
1. The manufactureres have not made enough for demand
2. With the exchange rate being in favour for exoprters from the UK, a lot are being exported abroad as the drug companies can get more money
3. There is a actually a problem with the manufacture or quality control of a drug
4. The manufacurers put a quota on a pharmacy for the month and if the pharmacy has had their quota of a drug then we cannot get anymore until the next month.
As others have sugessted, contact your surgery and ask if there is an alternative, also ask at the pharmacy if they have tried to get them directly from the manufacturer as often we are able to get enough direct, to cover a prescription although this usually takes a few days.