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The button has 2 levels of pressing, first (half press) it will focus, and second (full press) will capture the photo. You can go to Settings -> About phone -> Diagnostics -> Test -> and run a test on Camera button.
This might suggest it's a hardware problem, but last this try to go into safe mode and check if the same problem persists.
You just have to press it harder. Thought I have that issue but apparentlly I just didn't know I have to press it harder.