Low resolution raster images are one of the most frequent causes of poor raster to vector conversion results.
As stated in another article, raster images are made up of little squares called pixels.
The number of pixels per inch of a raster image is called its resolution. Resolution is measured in dpi (dots per inch). An image with a resolution of 300 dpi has 300 pixels along each inch. An image with a resolution of 600 dpi has 600 pixels along each inch, etc.
The resolution of a raster image is very important for raster to vector conversion because it determines how well the shapes on the image are defined. The better the shapes are defined, the better the raster to vector conversion will be.
For example, consider the curve below.
On a raster image, the curve must be represented by square pixels.
The images below show the pixel representation of the curve at lower and lower resolutions.
At the higher resolutions, the pixels are small and are able to represent the shape of the curve well. If you were to vectorize these images, you would get a vector curve.
As the resolution gets lower and lower, the pixels become too large to be able to represent the curve and no longer form a curve shape. If you were to vectorize these images, you would not get a vector curve, because there is no curve shape on the image.
If you want to do a raster to vector conversion, you must make sure that the resolution of the raster image you want to vectorize is high enough to adequately represent the shapes on your drawing.