Are you certain that it is not an issue with one of the switches or one of the gbic modules??
Couple of things you need to try here if your certain its the fibre.
1. Remove the connector, and with a lint free cloth gently clean the end of the fibre using a very small amount of ethanol (from your science techs, pure ethanol, not the stuff they mix with water to dilute it for kids to use) do this on all ends.
2. With a ear bud apply a small amount of ethanol and gently clean out the receiver end on the fibre panel to remove any unwated artifacts.
3. Call the people who installed the fibre to come and run some test on the continuity of the connection as it may have been damaged somehow.
Unless your skilled in the dark art of fibre then don't mess too much with it, get a professional to sort it.
D