it will be one of two thing's.
1 A worn key
2 a worn barrel
Both very common problems with ford locks. Go to a key cutting place and get them to cut you a non chipped key. If that key then opens your locks ok you need a new chipped key made if not you need to get the lock repair kit from ford, its about 20 pounds ish and they should build it to your key code for a small fee and just replace the lock.
http://www.fordscorpio.co.uk/doorlockbld.htm
ahould clear things up for you
Baz
May 2008