Sometimes, like everything else these days, they just fail. That button is meant to be pressed many times over without failing. One thing I would do with the new one is to put some electrical tape over the speaker hole. It doesn't diminish the sound noticeably but it does keep dirt and water out of the hole, therefore protecting it a bit. Sometimes dirt can get in the hole and cause all sorts of issues.
I know I'm late with this thread but I have to send the newest ProPointer in for some work as well. This was bought as our back up PP and it hasn't been used all that much. I decided to use it this year and what is happening is when I turn it on to locate a target and I get the beep, it just keeps beeping and going crazy. Yes I have changed batteries, made sure of the correct polarity, and the foam cushion was in place. No change. One thing I noticed when this starts is if I take my hand and give it a rap on the side it quits beeping so there has to be something else going on with it. I'll be shipping it back to Garrett to see what they can do with it.
I sent mine in in 2014 and had it repaired. I used it three times and it started falsing again. I called Garrett 3 1/2 weeks ago and was told that it would cost $21.50 to fix because it was out of warranty.. Garret called Wednesday and said it would cost $55 to repair. I told them to send it back as I can still use it. If it gets worse, I'm going to try and fix it as shown on youtube. I really like the PP