Fast-Acting Elixirs, Potent Elixirs, HGH all those affects “Elixir” trait skills, howewer they do not work for Elixir gun skills with “Elixir” trait and Elixir X. It seams that not all skills with “Elixir” trait in tooltip are elixirs. You can see complete list of elixir skills on gw2 wiki http://wiki.guildwars2.com/wiki/Elixir
Also i am playing hard with elixir build, can you post info about repairing this issue after confirming it ?