i've been testing FEC and how low we can go with maxavgbiterate

<param name="use-vbr" value="1"/>
<param name="use-dtx" value="1"/>
<param name="complexity" value="10"/>
<param name="packet-loss-percent" value="15"/>
<param name="asymmetric-sample-rates" value="1"/>
<param name="bitrate-negotiation" value="1"/>
<param name="keep-fec-enabled" value="1"/>
<param name="use-jb-lookahead" value="true"/>
<param name="maxaveragebitrate" value="14400"/>
<param name="maxplaybackrate" value="8000"/>
<param name="sprop-maxcapturerate" value="0"/>
<param name="adjust-bitrate" value="1"/>

we only see FEC if we set maxavgbiterate to 18000, we see no FEC with 14400

anyone knows what i'm missing?