The performance and reliability of your network infrastructure are only as good as your cables. Small variances can drastically affect network performance and can lead to downtime.
You need to be sure of the repeatability and reliability of your connections. There are industry specs (including assembly/connection loss, insertion test for reliability and functionality, pull test, etc.) that your cable manufacturer should adhere to - demand the documentation.
The mechanical characteristics of your cables are important. Make sure you are using cables built with high quality materials. Using inferior components can lead to poor alignment, which can lead to light loss. It is best to use factory terminated cables, as field termination can lead to inconsistencies.
Insertion loss/return loss of your cables is also critical. CABLExpress LC connectors have a maximum dB loss of 0.15 - well below the industry standard of 0.5dB.
Make sure you are receiving test results with your fiber cables.