If you are on the latest firmware (current firmware handles this much better), then it Could be two other separate issues: …
If on a Prime, go to the URL http://IPOFTUNER//lineup.html?show=all
Channels that are crossed out on the list (if any) are ones that either you are not subscribed to or the cablecard is not activated correctly (or the correct lineup is not assigned to it; if you believe you are subscribed to the channels, call your cable co.).
Channels that say “DRM” on the far right are protected and cannot be viewed in Channels or Plex. Those channels require WMC, DTCP-IP, the old SD Green Android app (no longer available and does not work on latest android) or Xbox One/Win10 (currently for live only; and very specific hardware requirements on win10)
Also, Plex does not fully adhere to the tuner sharing methods used by other software that follow the SD APIs for tuner sharing that allows them coexist with each other. Plex allocates the tuners upon initial scheduling and grabs all the tuners that the device[*] has available for its own use; initially it was only able to use one device (may have changed in the latest version update… not sire). This may make plex use “incompatible” with Channels or other software that use and require the tuner sharing API from SD unless certain measures are taken.
[* in this example, a connect device has 2 tuners, a prime has 3 tuners]