how can i fix my sewing machine-deltaNM902?

i cannot take up lower therad, the needle doesnt seem to go low enough and when it finally does the sewing machine doesnt sew! it is moving and all but doesnt do any stiches...it is a delta NM902 bought maybe 2yrs ago in aldi. i sew that some people have similar problems with this machine too...you advised somebody to retime it, could you please tell me what that means and how to do it.
thank you very much.
lucia.