This video helped me a lot:
http://www.youtube.com/watch?v=Q1mRhcquZTM
It seems to me your bobbin tension is too tight, which is why the upper thread is unable to pull the bobbin thread up far enough. There could be lint underneath the bobbin spring (video shows you how to clean). Or, with the change of threads, you may need to follow Jamie Wallen's method of setting the bobbin thread as loose as possible first, then tightening the top tension. That is what worked for me.