Bobbin in the wrong way is a classic, particularly for horizontal drop-in bobbin cases. It's easy to get this the wrong way, going from one machine to the other it can be oposite. The same for threading, machines can be very similar, but might have just one small but important differece along the thread route. I'm guessing the needle threads front to back on your Oekaki? If it has fancy functions make sure it's not a setting error.
Some machines are difficult to make behave with top stitch thread, but it's usually tension issues, not skipped stitches. What type of thread is it? Sew all weight Gütermann is my favorite for test sewing, or another trusted brand. Make sure there's nothing wrong with the needle, it's not very common but even a brand new needle has turned out to be flawed, even a bad package of needles some times.
I can't think of anything at the moment, it's usually sorted out by going through a list of points to check.
Yet another thing to check is cleaning, around the bobbin case, race hook and feed dog area. In the worst cases hard packed lint has cause all kinds of problems, including skipped stitches.