Every type of fabric has its own unique charactoristics, and its own challenges. Knits are no exception, needle type, backing, hooping, thread tension and proper digitizing all need to work together to produce the perfect end result.
Needle
To avoid cutting the fibers of the knit use a ball-point needle, a 75/11 should work well for any weight knit.
Backing
Tear-away backing is always the prefered choice as it is the least labor intensive to remove. however some knits are too delicate and can tear during the embroidery process so you need to turn to a no-show or organza backing which will afford more support without adding more bulk to the garment.
ToppingIf you are embroidering on a knit with a patterned texture such as pique, herringbone, birds-eye or cableknit you probably want to use a topping to flatten out the surface before you embroidery upon it.
Hooping
you do not want to stretch to knit in the hoop (with the exception of Spandex, which should be slightly stretched) you want to maake sure that your backing is tight in the hoop and the knit lies flat on top of it.
Tension
Because knits are less stable than woven fabrics they will cause satin stitches to pull in more and become thinner, especially if your thread tension is too tight. It is important to set your top tension looser than you would for a woven fabric and make sure that you bobbin tension is also loosened to make sure they are working together.
Digitizing
When digitizing for knits underlay and increased pull compensation are very important concerns. Use a lattice underlay under fills to minimise distortion and perpendicular underlay under satins to create a good base for the top stitching.
Embroidering on knits is no more difficult than anything else. If you understand why problems may occur they are much more easily remedied.