Parts can wear out, but that should have been caught when the machine was serviced. Service is typically guaranteed for a number of days (60 or so?) so, if it was recently serviced and you still have this problem, I would take the machine back under the service warranty and tell them exactly what the problem is. Provide a sample sandwich that shows the problem (and that they can also test on).
You shouldn't need to raise the feed dogs. You can try lowering the presser foot pressure if your machine offers a way to do this (not all machines do).
Walking feet can wear out too. It is usually the spring inside the walking foot that finally doesn't "spring" back as much as it should. If you want to try replacing the walking foot, it usually helps to buy a foot made especially for your machine. Generic walking feet often don't work as well. I doubt that you would have worn out the feed dogs on the walking foot; however, you could have worn out the spring inside the walking foot.
I assume you are talking about the quilt sandwich not feeding the way it used to? Does the machine still feed correctly for piecing? If so, I am thinking it's likely to be a worn-out spring or worn-out machine part that is not working at 100% anymore. If the problem is worse with the walking foot, I would think you have both problems -- a worn out part in the feed dogs and a worn out walking foot.
Try Googling your Janome model and see if you can find any Youtube videos, diagrams, repair manuals, and/or troubleshooting websites that address feed dog issues. It's possible to do feed dog adjustments on many vintage machines, but newer machines often don't have an easy way to do it yourself.