The Unicolor drums leak to varying extents, depending on the condition of the gasket that should run around the edge of the inside of the cap. Does your cap have a gasket? If so, is it lying flat, or does it bulge in one or more spots? If it doesn't lie flat, you may need to replace it.
Also, you must make sure the cap is really closed. This usually requires quite a bit of effort, and you can tell when it's "snapped" shut. If the feet of the drum are not perfectly aligned, then the cap is not closed.