Keep setting your timer and peaking in on your work every five minutes or so to see how the felting is going. You might need to reset your machine to start the agitation cycle again once or twice, depending on the length of the cycle and how the felting is going.
Every piece of knitting is different in terms of how it will felt. Some colors of the same brand of yarn felt differently, and its a common experience that white or very light colored yarns are more difficult to felt.
It is important not to let your washer drain or spin during the felting process, so make sure you keep an eye and an ear on the machine and reset it when it starts to drain. Just stop the machine and turn the dial back around so it begins to agitate again.