JPEG files are unsuitable for most raster to vector conversions.
Unfortunately, many scanner users are unaware of the downsides of JPEG files, a problem exacerbated by the fact that many scanners default to color scanning and then default to saving the scanned file as JPEG.
In addition, a lot of people expect to be able to download JPEG (and GIF) images from the web and vectorize them.
So, why are JPEG files a problem?
JPEG uses "lossy compression". When you save an image as JPEG, some of the image detail is lost in order to make the saved file as small as possible.
The image above shows the type of line quality you need for raster to vector conversion.
The images above shows the type of line quality you are likely to find in a JPEG file.
As you can see, the lines in the JPEG files look blurry and contain JPEG induced artefacts - "clouds" of gray speckles around the drawing lines.
This means that JPEG files usually require extra raster cleaning steps before they can be vectorized. Some JPEG files can never be cleaned to the point where they can be successfully vectorized.
Note that once you have damaged an image by saving it as JPEG, you cannot undo the damage by simply converting the JPEG image to a high quality file format like TIFF. You need to rescan the drawing.
Rather than waste time cleaning or rescanning, get it right the first time. Save your scanned black and white drawings as TIFF Group 4 and your color images using a lossless color format such as packbits compressed TIFF.
A black and white drawing scanned in color and saved as a JPEG file will be many times larger that the same drawing scanned in black and white and saved as TIFF Group 4.
Many JPEG (and GIF) files that people try to vectorize (particularly logos) have actually been created for display on the web.
The image characteristics required for web display and raster to vector conversion are completely different (see Why are web images usually unsuitable for raster to vector conversion?).
For example, the logo above has been created for web display. Because it's been created for web display it looks fine on the web.
However if you zoom into logo, you will see it is blurry and stepped (the image above shows the letter "r"). You will not be able to vectorize the logo.
In order to vectorize the logo, it needs to be crisp and smooth like the image above.
Note that you cannot create a crisp and smooth image by taking a JPEG or GIF image created for the web, loading it into something like Photoshop and increasing its resolution. In order to create a crisp and smooth image suitable for raster to vector conversion your original artwork needs to be large (if hand drawn and scanned) or high resolution (if created electronically).