if there is too much drag behind a jet then they will roll over i.e. running a stick bait or sometimes crawler harness. this generally happens with the 20's or sometimes 30's. sometimes you will get a jet that is defective and there is nothing you can do about it. i have never found any physical reason for them not to work but some just don't.
You should check the "tune" of the jet before setting just like you would a crank. Also, you may need to "round out" the sides of the hole where the snap goes thru the fin of the jet. Take the snap out and use a sharp, pointed knife to remove a little plastic around the edge of the hole. Right out of the package, the connector can get caught slightly and not allow the jet to run true.
Here's a link to Jan's Netcraft, and the connectors I use. http://www.jannsnetcraft.com/connecting-links/335505.aspx
You still should bevel the edges of the hole, but these making tunning the Jets much easier. You need the one with different size loops. Then you can tune the Jets with your fingers.
while were on jets , i have 1 30 jet that will not flip and dive??? it just stays upside downand on the surface,even if i try to put the inside weight in the down/dive position. should that weight stay in the down side of the jet?
Make shure that the throat dimension is wide enough that it will let the jet settle in to the radius of the snap. If the snap is too small it will bind and send the jet to one side or the other.
I use the same size snap on the main line for all types of divers;don't rember the size at the moment but gauge its size for the jets as they are the toughest hook up and run true.