Sounds like the hook/needle timing. To check do the following:
With the power off!!
remove bobbin and bobbin case.
push needle all the way to the left.
Turn handwheel toward you watching the needle and bobbin housing hook. The point of the hook should line up right above the eye of the needle.
If not timing, clean bobbin area removing lint and thread.
Check bobbin tension, should get slight resistance when pulling thread.
Make sure using only Husqvarna Viking bobbins -- most are green but they have new colors now. All will have the Husqvarna logo on them (an H). Make sure the bobbin is unwinding in the right direction.
Remove fabric. Pull extra long bobbin thread (about 3 inches). Hold upper thread tail in left hand, turn handwheel toward you for 1 revolution. Is the bobbin thread caught by the top thread and pulled through the opening of the needle plate?
If you are still unable to get a good stitch, take to an authorized Husqvarna Viking dealer for service or have them send to Husqvarna Viking (SVP) Technical Service Dept for repair. A list of dealers close to you can be found at the Husqvarna Viking website