Unfortunately, signal noise has a tendency to introduce errors during computation. This can actually alter and corrupt the state that a bit (and by extention, qubit) is in, thereby corrupting whatever representated data that bit is a part of. A weakness of quantum computing is its obvious reliance on a basic principle of quantum physics, which make redundancy and repetition (a simple matter in conventional computing) impossible as a method of error correction. Researchers have also found that their current method of error avoidance (as opposed to the reactive error correcting) cannot be used on its own, dashing any hopes of using that method exclusively in future implementations of the quantum computer.
Then again, whatever that is determined to not work only narrows down the possible choices of what will work, so research continues to advance.