Google runs a bot guard script that'll track everything right down to mouse movements and the speed you type at. To give you an example, a friend of mine made a new account creator for google not too long ago and all the accounts were getting banned after creation. After trying a few different things with no noticeable difference in the results, he added one more thing where he literally just made the mouse scribble side to side and around on the page. Now obviously we already had the typing and mouse movements emulated and with random pauses to look like a normal person but they were still dying. Once he had the mouse do that scribble movement they started staying alive perfectly. Lol. Now that's not a guarantee in any way, but in our case it made all the difference. In yours it might be something else, but there's a lot to consider with Google.
I also think that certain things can trigger tighter security. So say for example you are using a really clean IP that Google hasn't seen before, that mouse scribble movement stuff, and typing emulation and other things we had to do might not of been needed then. If you're using a known proxy though, they're probably stricter the other things.