Tripwire functionality is orientation dependent: I reviewed the wiki and it quoted Dinnerbone saying "there either needs to be no blocks under any wire/hook or all blocks under the wire/hook"; however this is not actually the case. Since this "mix and match" is based on whether or not the underlying block has a transparency value, any block with a transparency value can be "mixed and matched" with air."

However I found that with one block of air below the hooks and tripwire, if I placed a solid block under the East tripwire hook, the tripwire would still activate, but if I either a.) place a block under the West tripwire hook, or b.) remove the East block and also do (a) then the tripwire will not respond when you walk through it.

If you swap this from west/east to north/south then putting the normal block on the South end has the same effect as on the West end in the previous example.

I also noticed that when doing (a) the tripwire appears to be fine (straight; indicating a "good connection" between hooks) until you walk through it once, and then the hook bends up and it won't respond anymore.

This does not happen when using transparent blocks so it seems like the "no blocks / all blocks" check isn't happening depending on orientation.

This happens in Creative as well as Survival - I also attached a screenshot of my test setup.