There may be several reasons:
You can try to fix this problem by yourself (so we can be focused on improving wallabag internally instead of writing siteconfig :) ).
You can try to see if it works here: http://f43.me/feed/test (it uses almost the same system as wallabag to retrieve content).
If it works here and not on wallabag, it means there is something internally in wallabag that breaks the parser (hard to fix: please open an issue about it).
You can test it on f43.me website: click on Want to try a custom siteconfig? and put the generated file from siteconfig.fivefilters.org.
Repeat until you have something ok.
Then you can submit a pull request to https://github.com/fivefilters/ftr-site-config which is the global repo for siteconfig files.
If wallabag failed when fetching an article, you can click on the reload button (the third on the below picture).