If the sitution is as described then the obvious answer is yes. People don't change, so attitude issues are hard to rectify once the settle (certainly not in the timescale of a young startup).

That said, the big take-away shouldn't just be to change your hiring process but rather to (also) change your management process. I find that staff attitude is set by both sides during the first few months of employment. At that point it "locks in" and is much harder to change later.