Well, the reasons for the canvas to show through thick brushstrokes could be many.
Here are three situations that come to mind:
1. Some cheaper canvasses behave a little weird. Even if they have been primed with gesso, somehow the paint seems to slide on the surface as you apply it and sections of the canvas will show through. My solution to that is to paint a base coat of paint as ground color and let it dry. Further layers of paint will have a better grip. Another solution would be to add one or two layers of gesso to the canvas.
2. It could be a low-quality paint issue. If you try to paint with craft acrylics or with some student-grade paints you may have to fight against the lower pigmentation and the fillers of the paint. The solution is to buy professional paint, look for artist quality..
3. It could be that you are using transparent paint, in that case the paint it's only doing its job, you may want to learn which colors are more opaque and use them accordingly. Most acrylic brands include an indication of transparency/opacity on the container. Opaque colors mixed into transparent ones will lower the transparency.