That sounds like a problem with the top tension. Top tension is not tight enough to draw bobbin thread to middle of fabric. If you have some *unwaxed* dental floss, you can floss your upper tension between the discs to get rid of any lint or thread piece that might be interfering with tension. Also, try increasing your upper tension.
For the bobbin case (assuming you have a removable bobbin case), you can take a pin and *gently* sweep with the pin underneath the tensioning piece. Sometimes lint accumulates there, and the pin will remove it. Although the following tension video was created for longarms, the same principles apply to domestic machines, and he shows the pin thing with the bobbin case:
http://www.youtube.com/watch?v=Q1mRh...664A7&index=13
Also, did you remove the needle plate and clean any lint out from under that?