Looks like you have not read the manual on both camera and lens.
If you give more details like the setup of your kit, are you taking a shot with the subject too near, light condition, etc.
i think people will then be able to give some tips.
but.... but... if you half-press the 'trigger', how would the camera be able to capture a shot?
The shutter only activates upon a full press of the 'trigger' (shutter release) button :angel:
anyway, catchlights has already answered your question.
The camera can't achieve focus lock in autofocus mode, so it won't fire.
If you switch to manual focus mode, it will fire regardless of focus. But then you run the risk of focusing being out by a bit.