In knitted garments ridges that occur in the knitting are referred to as "ladders." They can occur when you are knitting on the round, both when you are using a round needle and when you are using four double pointed needles. This is a very common problem that even the most experienced of knitters encounter. The ladders occur in the places where the two ends of the needles come together. If you are knitting socks, you could have four ladders extending the length of the sock and when knitting a larger garment, such as a sweater on a round needle, you will have one ladder at the point where the needles meet in the work.
The tension of the work is not of the same consistency at the meeting point of the ends of the needles. This is what causes the problem of ladders to occur. Although it looks like you have dropped a stitch all the way through, there is no dropped stitch. It appears that way because of the loose tension when you begin each round of knitting.
The problem of ladders is more noticeable when you are using four double pointed needles, using three to hold the knitting stitches and a fourth to work the rounds. This happens because there the three holding needles give the work the look of a triangle instead of a circle. Because there is such a stretch between the last stitch on one needle and the first stitch on another, the work is at a steep angle and the wool is carried across a much wider space at this point that when all the stitches are on the same needle. It is often harder to knit the first two or three stitches of each needle than it is to knit the remaining stitches to the end of the needle.
One tip for solving this problem is to use five needles for the project instead of four. If you divide the number of stitches on four needles and use the fifth one for knitting, there is not as much tension between the stitches at the ends of the needles and this will eliminate the problem of ladders. This is because these stitches will be closer together and therefore won't be as hard to knit because there will be less tension on them.
In some cases, knitters do not pull their wool tight enough when they change needles, which means that the first stitch on the needle is looser than the others. This leaves a loop at the beginning point of each needle in the round. Sometimes, knitters who do recognize that tension is the cause of ladders in the knitting pull the wool too tight when they start each needle and this results in ladders as well because the tension on the first stitch is too tight.
You can compensate for the difference in tension by knitting looser if you find that you are pulling the wool tight at the start of each round. You do have to think about your style of knitting because if you already knit loosely, you will have to pull the wool tighter to compensate for this.
Another way in which you can solve the problem of ladders in your knitted work is to change the point at which the two needles meet after a few rows. Since the work will be all in one piece when it is complete, you can easily do this by continuing to knit on the same needle for a few stitches each time to come to the end of a row. Use a small piece of wool of a different color wrapped around the needle to mark the beginning of the row.