[atarget]

The shutter must be depressed (half depressed when using the camera´s shutter release) during the time autofocus takes to lock. In "normal" mode, PDAF is fast it locks before you let go the shutter release. But in Liveview, CDAF takes longer, so you should hold the shutter release until the "focus square" on the screen turns green or you hear the focus confirmation sound. This applies whether using the cable release or shutter release on the camera.
That´s why sometimes it fired and some not, you weren´t holding the shutter release long enough. Disabling autofocus when using liveview on a tripod seems like the best option for me.
Also, when focusing, liveview magnifies the image automatically to the zone where it will autofocus on.

I only use LV when camera is mounted on tripod. In hand shooting situations there's too much image instability to take advantage of LV ( IMO). Because - on the other hand - I use LV only when employing manual lenses, it's quite natural to disable AF beforehand for me ( the only instance I use AF-S with manual lens is when I hand shoot still or slowly moving subject and I take advantage of nice "fully depress shutter release button and let the camera focus" trick )
Again, I would like to point out that I'm not 100% happy camper with my K-5's LV mode due to graininess of blown-up image. And it practically happens at ANY ISO setting ( not just high ISOs). My basic Samsung NX 1000 does better job in this department giving me crystal clear live view magnification ( up to 8x only but it usually does the trick) so I can precisely hand- focus.