Right, try again...I'm sure it said home button but there you go, I'm an idiot!
Power button...if it feels like it is sinking, and you've replaced the physical (outer) button, check you've got the right screws in. These are quite small, small headed screws, which I've got wrong before and had that sinking feeling. Make sure the silver loop on the inside of the frame (ie the back of the power button) is sitting on the ledge in the rear case(it acts like a spring and pushes the button back out again). If you've over tightened the screws generally it puts stress on those holes and it can crack and become loose (sinking feeling) . If you have only replaced the headphone/volume/power flex and still have the sinking feeling then I'd suggest your problem is the outer power switch has a crack in it.
Having a loose bezel/outer frame on the phone can also interfere..just put a bit of plastic friendly glue in the upper right corner and press frame and case together. I've only had this with replacement cases or dropped phones not general wear and tear.