If that's the case it maybe that the Bluetooth version on your headphones is an old version and not compatible with the devices you are trying to connect to that may have a newer version.
Devices with a Bluetooth version of 4.0, 4.1, 4.2 & 5.0 may not be compatible with headphones that have a Bluetooth version of 2.0.
The later versions are backwards compatible.
Did they say it will connect to ALL devices, irrespective of the Bluetooth version?
First thing to do is ask the factory what Bluetooth version is used on their product and if it's compatible with all Bluetooth versions.
Once you have this information, you will be better placed on how to handle the problem.