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As stated in the introduction, generic PLC can only be used on slin audio. The majority of audio communication is not done in slin, but rather using lower bandwidth codecs. This means that for PLC to be used, there must be a translation step involving slin on the write path of a channel. This means that PLC cannot be used if the codecs on either side of the bridge are the same or do not require a translation to slin in order to translate between them. For instance, a ulaw - ulaw call will not use PLC since no translation is required. In addition, a ulaw - alaw call will also not use PLC since the translation path does not include any step involving slin.   One item of note is that slin must be present on the write path of a channel since that is the path where PLC is applied. Consider that Asterisk is bridging channels A and B. A uses ulaw for audio and B uses GSM. This translation involves slin, so things are shaping up well for PLC. Consider, however if Asterisk sets up the translation paths like so:

...

In this scenario, the write paths for both A and B begin with slin, and so PLC may be applied to either channel. This translation behavior has, in the past been doable with the transcode_via_sln option in asterisk.conf. Recent changes to the PLC code have also made the genericplc option  option in codecs.conf imply the transcode_via_sln option option. The result is that by enabling genericplc in  in codecs.conf, the translation path set up in Fig. 2 should automatically be used as long as the two codecs required transcoding in the first place.

If the codecs on the inbound and outbound channels are the same or do not require transcoding, PLC won't normally be used for the reasons stated above.  You can however, force transcoding and PLC in this situation by setting the genericplc_on_equal_codecs parameter in the plc section of codecs.conf to true.  This feature was introduced in Asterisk 13.21 and 15.4.