To bring the rear der inwards (more to the lower gears or larger cogs) you do what is actually lengthening the cable casing. If the inner cable was fixed at both ends then it's tension would increase too. But since the cable is attached to a spring loaded end (the der parallelogram spring) the "tension" won't change much. What will change is the rear der's position WRT the controller's "click point".


This adjustment, what I call an index adjustment, is one of the most basic ones we do many times a day. It's an adjustment that requires no tools (we hope your bike has threaded barrel adjusters) and one that can be reversed as easily as it was changed. The key is to remember which way and how far you turned the barrel. If you can't remember then write it down. Seriously, because even pros will turn the barrel the wrong way sometimes.


So go ahead and try. If you can see the ends of the fixed and the rotating parts of the barrel you can actually see the increased length of the barrel's overall length. After making a barrel change, that is recorded/remembered, pedal the bike to see what result is gained. This process of trial a bit of barrel change then test the bike is far easier to do if the bike can be stationary and the rear wheel off the ground. (A loop of cord around the seat and a ceiling joist, a trainer stand with the resistance off, a repair stand, a friend) so you can pedal and also closely watch the der moving the chain over the rear cogs. The goal is to have the der pulley directly below each cog after each shift has been done. A slight push/pull on the der, when at this balanced cable adjustment, will start to scuff the next larger cog or try to climb off the cog the chain is on. Release the push/pull and the pulley re centers under the intended cog. If this is all done and using the controller (lever) but you can't shift into the lowest gear then your index adjustment is likely one whole cog off. So just reverse what you did. This is why you want to remember what you did, how much of how many barrel turns, so you can go back and try again.


Cable caused ghost shifting usually is a slowly degrading process. Ride by ride the cable/casing system is changing bit by bit and at some point the collective bits are enough to cause a skip due to the pulley no longer being directly under the intended cog. A sensitive rider can often feel this degradation before skipping has started and might only need to turn the barrel less then a full turn to keep up with cable "stretch" (actually casing compression). A rider who wasn't so sensitive or proactive might need to turn the barrel more then a turn, two or more turns suggest an extreme case.


If the skipping came on suddenly, and the rider is sensitive, then there might have been an incident like a fall down of the bike (I've even fixed bikes that were impacted by soccer balls and trash cans) may have happened and the entire rear der might have been bent inward. If this has happened then besides the, now, misalignment between the intended cog and where the bent der places the pulley there's also the chance that the der can now overlap with the spokes when shifted to the large cog. Not good! Most modern bikes have an attachment tab called the hanger that is purposely weaker then the frame, can bend under relatively little pressure and can be either realigned or replaced as needed.


There are other reasons that a bike will ghost shift but with a new bike these other causes that are extreme wear related are not likely to be at play (yet).


The OP said the bike is new. cable settling in is expected on a new bike. This is but one reason that LBSs will offer a free after sale service at no cost, to retune the bike after initial break in periods. If the bike was bought at a shop then I strongly suggest taking the shop up on this offer. Because there's a number of other aspects of safe and sound mechanical function that also are bedding in and should have an experienced hand correct/confirm their condition/tuning/adjustment. If the bike wasn't bought from a LBS, or the shop sucks and doesn't offer after sale service then the single best source for mechanical help is possibly Park's Big Blue Book. Andy.