Hello.
I have been trying to brew kombucha for about 3 months and have had the same problem. No new scoby, just scum. The tea tastes as if it has fermented and the third brew even got fizzy. But, no new scoby. I have been reading and is seems this can be caused by starting with a dehydrated scoby. I am ordering a full size scoby and will let you know if I have better luck. Hopefully you have not given up on your kombucha brewing.