Power button not clicking and barely working

Is it the power flex you have replaced or the actual button? If it is the flex, it may not be seated quite accurately enough for the button to "click" it properly. It only needs to be a millimetre or 2 out of place to not work properly.

Strip it back down to where the flex is accessible and see if the clicker on the flex actually clicks and bounces back out. If it does it just needs resituating. If it doesn't it's a fault with the flex.

If you have just replaced the button, it may be that it is not moulded accurately and you may just need a new button that hits the clicker more centrally