When crocheting in a circle, if there were never any increases, the crochet would form a tube.
To make a flat circle, you need to increase stitches. See my other post for the formula: http://craftcove.blogspot.com.au/2017/12/making-flat-circle.html
There is a formula to create a flat circle, but not all patterns use the formula, or use the formula the same way. Or your tension may be different. This may cause the circle to go wrong.
Sometimes it's actually part of the pattern design, and nothing to worry about.
The main problems that can happen are waving or curling
If your circle is all wavy like the images above. The problem is that the outer edges are increasing in size more than needed.
This can happen if there are too many increases in the rows.
The answer is to reduce the increase stitches in each row. You could also try a smaller hook in the outer rows to try to "tighten" it
The above image shows the waving from earlier on. Try to fix it before it goes too far
Curling is when the edges curl up into a bowl. The problem here is that the outer edges are not being increased enough.
To fix this problem, you need to increase some stitches evenly along the rows.
Sometimes a larger hook can help if used along the outer edges
Next week I'll talk about others things that can go wrong with the circle