OTN and SDH devices fail to be interconnected

Dear friend!
The possible causes are as follows:
Cause 1:  The line between the WDM equipment and the SDH equipment is faulty.
Rectify the line fault, such as great attenuation or fiber cut and so on.
Cause 2:  The parameters of the client-side port on the WDM board are incorrectly set.
Check parameter settings.
Cause 3:  One device uses a single-mode optical module, and the other device uses a multi-mode optical module.
Use optical modules in the same mode.
Thank you!