All 4 of the buttons should feel the same - A 'snap' when pressed in and they pop back out when released. It's very common for the power button spring to collapse and that makes the Key useless. They can be rebuilt . . . . some have opened them up and done it themselves with various degrees of success
One of the I2's we bought had a Key behaving pretty much as you describe. I sent it to a guy in Texas that I found on eBay to have it rebuilt. He told me that the springs for the keys can't be bought new anywhere - Segway doesn't sell them, even to dealers. He cannibalizes old broken Keys for the parts to rebuild Keys . . . . he has dozens (maybe more) to pick parts from. He also can reprogram a used Key to work with your machine is you give him the serial number
Vince at
www.texassegway.com was the guy I used
Don