I have a Gammill longarm and I use strictly Singer needles that are made for the longarm machines, they have no flat side at the top but they do have a concave section at the bottom. The Gros-Beckett needles are made the same. It could be the needles you are using are for regular machines and not quilting machines, which sew differently. I suggest calling your Viking dealer and get needles specifically for the mega quilter, also check that you are threading it correctly.
Longarm