I find the diffusion excellent, actually - watching the way the very thin sheets of vapor and water mix pressed between the tubes - it's really a well-designed perc and I believe G designed it himself. Turning it around in my hands slowly while tilted and hitting it, the "active" hole changes to whichever one is near the top. While holding it fairly level it seems random which one will fire next.
How clean is yours currently? I take some 99% ISO (lower percentages but above 88% are ok, just takes a bit more elbow grease and shaking it to get same effects) and add a bit of table salt, hold it between my thumb and ring finger and shake it, shake shake shake shake it, shake it like a Polaroid pict-cha! Followed by a good long rinse (making sure to blow it out every so often because it's designed to RETAIN, not flush water
)
If there's a hole (I count 5 on mine - the small dot looking things just above the lower ridgeline where the glass expands above the metal mounting spacer) not firing after you're certain it's perfectly clean, give D9V a call and discuss it.
Personally it doesn't bug me much if a hole or three on a much bigger perc isn't firing as long as the glass looks nice. Much more than that and it starts negatively impacting the draw because it becomes much more restrictive if there's only so many working holes. This is exacerbated in the Nibber X's situation.
Unless you know a buddy with another 100% working Nibbler X is you can physically compare it to, it's really hard to gauge if your draw is harder than designed. On mine, the water level itself really affects the drag, and with minimal water (like 4-5 drops) there is still a bit more drag than compared with a cartridge with a mouthpiece.
One tendency when encountering an air restriction is to try to suck golf balls through a garden hose like a champ. This is when you laugh at your friend after he winds up with a mouthful of oil after he just nuked your 1.5 cart at 7.2v. Sad soldier burial time, but he deserved that for wasting my shit - and no you gotta go home and use your own Scope, asshole.
The Nibbler X quickly breaks you of that habit by filling your mouth up with water before the cart starts spewing. It sets the pace for what the fastest draw you can do and trains you for when you're using a cart alone. It does not necessarily teach you the OPTIMUM draw speed, however, but it's still a great state to be in.
So ultimately I would ask myself if the draw was so restrictive that a less-experienced friend winds up with mouthfuls of water no matter how hard he tries, and/or if he can learn to draw even slower to crank up a power cloud, milking and sexing that Nibbler X up so nice! That's the operating window for this perc, and if it's not comfortable to use in that range, I have an issue with it.
However, that is my operating criteria and not necessarily your own, so having a hole not fire is a perfectly legitimate complaint should you choose to make it one. I don't know D9V's policy regarding return/exchange since glass percs are a really funky subject with most outlets, but they have been outstanding and upstanding with myself (I had an order a bit back with some bad parts which they quickly replaced at no additional cost to myself.. can't ask for anything more).
Hopefully that sucker fires up for you and all it well. I'm really impressed by the technical craftsmanship on these pieces (Nibbler X and Bender) as well as the overall elegant lines and subtle light effects since the glass panes are perfectly symmetrical.
Give 'em a call and discuss! I'm sure they've been bored out of their minds since 4/20!