Not good enough information.
The fiber should be single-mode fiber, something like 9/125.
Length is important, especially if the fiber is too short. Imagine trying to listen to someone talking to you from two feet away with a 1000 watt audio amplifier ... it's painful, and you won't understand. After a while, you'll go deaf.
If your (single-mode) fiber cable is too short, the receiver is getting an extremely "loud" signal that will likely be distorted (i.e., receiving errors and not communicating) ... and it will go "deaf" after a while (receiver will burn out or de-sensitize).
To prevent that, you need in-line attenuation to bring the signal level down to a "comfortable" level for the receiver.
By knowing how much signal the transmitter puts out, and how much signal the receiver needs for accurate & reliable understanding, and factoring in how much attenuation you get from the length of fiber you are using, you get a number that you use to decide how much additional attenuation you'll need (or whether you need it or not).
You can get fixed attenuators (one value) or variable optical attenuators (VOAs) ... but chances are you'll need to reduce the signal somewhat (and soon, before the receiver is damaged).
By cleaning, I mean cleaning ... remove dirt, film, oils, scum, crap .... bad things from the place where the beam of light enters and exits the fiber.
Single mode fiber is 8.3 microns in diameter ... 1/10 the thickness of an average human hair ... it doesn't take much "stuff" to block or diffuse the signal and kill your connection.
There are special cleaning kits / materials generally used to clean the ends of the fiber .. it's fairly soft glass and scratches easily ... so don't lick it and rub it on your shirt to clean it.
So, again, how long is your fiber, what kind of fiber are you using, and are you using any in-line attenuation?
Most single-mode jumpers are canary yellow, many/most/all multi-mode jumpers are orange-colored.
If you don't tell us, we can't help.
Good Luck
Scott
