Just bought a Brother PE-770. The bobbin thread is not showing any on the backside of the fabric. Just the colored thread from the top. Adjusted the tension but no change.

Start threading the top thread.Make sure the Foot is up cos that will open the top thread tension discs, now hold the thread a little tight with one hand at the reel while you thread through and use a flossing type motion this will pull the thread properly into the discs continue until the needle,give another little back pull,now thread the needle.It will become your usual habit and never have the top thread loose again.
From Bev

Are you sure the bobbin is in the machine correctly? I had the same problem at one time and I had it winding from the right side instead of the left side.

Is that a combo machine - one that sews and does embroidery? If so, are you sure you are using the embroidery bobbin case (there are usually two different ones on combo machines)? And, are you using embroidery bobbin thread on the bottom? Finally, what kind of design are you stitching - satin stitching has mostly the top thread showing on the back and that is normal.