Under Extracted Shots can be caused when the espresso is too coarse. or if you don't pack enough grinds into the portafilter.
can you tell me what is the pressure? and temp? also while your at it let me know what is the water level of the machine?
Crema problems can be either the roast it self that your using or the machine.
could you tell me how long does it take your machine to brew a single shot? that would help out as well.
I'm asking because I've been in the business for a while not on your end but on the espresso machine repair, refurbish, sales, training end. So maybe I could help out.