I believe there are a few threads in the tips & tricks forum.
I have found that my dark juices are not fond of the vivi nova but my light juices taste exceptional.
#1. Try cleaning out your tank and vaping a light colored juice. See if it is better.
#2. Try trimming a small portion of the silicone tube with a razor blade to help with wicking. (See thread in tip & tricks)
#3. I was vaping a mini vivi, everything was great, cleaned it, put on a new atty, and it suddenly sucked. Such is the world of vaping today. Not sure what to make of it.
....so far I have had no problems at all with the 3.5ml tanks and light colored juices.