As far as I understand is standard Ubuntu policy to stop upgrading packages for non-LTS distributions as soon as new release is done.
Hence new packages will be updated only on new version of Ubuntu, but the new version is NOT compatible with plug.
Considering this you have 2 options left:
- monitor packages and as soon as new version is available compile source to have new version for your distribution (either cross-compile or compile in the plug)
- switch to Debian - and everithing will run smooth