Why is the link missing?

Question

Computer networks

Good day. I turn to you for help, because I need it.
There is a network based on fiber optic cables. Unmanaged Powertone PoE switches (5 in total) are connected to the D-Link DGS-3120-24SC managed switch. After commissioning, more than 4 months have passed and there have been no complaints, until now.
Link lost on two switches. These two switches are territorially located farthest of all, although the distance from the switch to the server room hardly exceeds 1 km.
Small network diagram:

My problem is with cabinets number 3 and 4.
When using an optical tester, the results are in the range of 4 to 4.5 db (much?).
The equipment from the cabinets where the link disappeared was installed in the working cabinets, where it continued to work. There were also manipulations with sfp modules, which also proved to work.
In the server part on the switch, the sfp modules were also swapped, which showed their operability. Also, as part of the practice, D-Link DGS-1100-10 / ME was used, on which the link also did not rise (there were suspicions about D-Link DGS-3120-24SC, because there are similar problems in its operation, judging by the forums ).
If I haven't provided any information you need, please let me know and I'll update it.
What is the problem, please tell me?

Answer 1

Friends, thank you very much for your help. Many tips have expanded my practical knowledge. The problem turned out to be absurdly simple. The fiber in the crosses was a little stretched, resulting in an angle through which the signal did not pass.
Please advise the materials on which you studied, or consider it useful in our business, I will be extremely grateful.
Thank you for not giving up!

Answer 2

cable ... or kaput junctions. hiphip hooray!