Check your NLog.config to ensure the layout includes ${exception} :
NLog.config
${exception}
<target xsi:type="File" name="logfile" fileName="log.txt" layout="${date} ${level} ${message}${exception:format=ToString}" />
target xsi:type="File" name="logfile" fileName="log.txt"
layout="${date} ${level} ${message}
${exception:format=ToString}
" />