Hi Jane,
Further to Don's response... when configuration management is enabled, linking is directional. One artifact "owns" and stores the link information as part of its attributes, and also publishes that link information in the TRS feed to LDX. The target artifact's application checks LDX for incoming links and displays those when you open it. The configuration context is required for the versioned artifacts. As the Knowledge Centre article explains, RTC "owns" any links from RTC artifacts, including work items, to any other domain artifacts (requirements, tests, designs). So the link is stored only in the work item, and the requirement has to get the link information from LDX.

Although RTC work items aren't versioned, if you want to link them to versioned artifacts (like requirements), they need to be able to understand configuration context. You need to define Releases, associate them to iterations, and use the link/attribute mappings to set up the association to global configurations. This video shows you more about that.
(You can add an RTC stream to the GC, but as you point out, that is for source code artifacts only. Work items don't have configurations because they aren't versioned.)

So make sure you have configured RTC correctly with the desired GC associations. Also make sure you're using the right attribute for the link type - for example, the default mappings use "planned-for" values for "implemented by" links, but "found-in" for "affected by". 
Make sure LDX is installed, and check LDX admin (https://server:port/ldx/web/admin/home) to make sure the TRS feeds are updating correctly. You can also reindex the RTC feed from LDX admin if anything looks amiss.

Hope that helps.