The gumming may be related to rapid drying. A small amount of retarder or thinner may help, as will raising the relative humidity in the workroom. There are, however, a host of other possible problems, starting with bad paint and including an unfortunate change in formulation. The latter you can do little about, except complain to the manufacturer. If they get enough complaints, they may fix the problem.
Coverage with many whites is often a problem. First, make sure that the paint has been thoroughly mixed. The pigment used in white (titanium dioxide) is relatively heavy, compared to some, and may settle harder than most. Undercoating white with silver will help a great deal. Also, flat whites tend to have better hide than gloss whites.
And I second Jon's endorsement of Polly Scale. I use that and Tamiya almost exclusively. Tamiya is a more difficult paint to work with, at times, but I like the results I get. Polly Scale isn't quite as good a detail paint for brushing, but it produces a wonderful finish and is a joy to use.