To make a good connection the connectors to the coils must snap together. I have found that sometimes the rubber gasket in the harness connector gets displaced (rolled sometimes) and prevents the connectors from snapping together properly. As a trouble shooting method, remove the rubber gasket from the connectors that are not snapping together and see it they will then snap together. You can probably run for years with the rubber gaskets removed but I would recommend replacing the rubber gaskets or the entire connector at your leisure.