What can I do to improve opacity?

Ink selection is very important. Be sure that the product you choose is listed as opaque ink. However, selecting an opaque ink will not ensure a totally opaque print. 

Factors such as print parameters and screen preparation contribute to achieving maximum opacity. When printed correctly, thicker stencils on screens will allow greater amounts of ink to be laid on the surface of the garment. The most effective way to print is with a hard flood that fills the stencil space completely, print with low-pressure and low angle with a hard, square squeegee. This print method allows the ink to stay on the surface of the substrate. 

Opacity is greatly diminished if the ink is driven into the substrate. You may also use additives in your ink that expand like High Density Additive or puff additive, this will however, diminish any smooth surface. Start with small amounts of additive until you are satisfied. There are many fabrics available today, and many more release every year.