As with most optical link problems follow the light and measure from both ends.
Verify the SFP module installed in the port - adjust the fpc and pic slot numbers to your interface
verify the optics on both sides are as expected for distence and wavelength
show chassis pic fpc-slot 0 pic-slot 0
Confirm the send and receive power on the optic in the MX - look at your interface
show diagnostic optics xe-0/0/0
go to the next device and verify the light recieved is with acceptable loss for the distance from the transmit value above at each stage on the way
when you find a suspect link with too much loss, shoot the fiber to find the flaws.