Sewing On Paper -----PROBLEM SOLVED Thanks to Willow Traders :-)

Okay so I bought a machine a Brother, thank you to those who gave suggestions. The Janome was virtually impossible to find here and once I did find it, I had already bought this one :-) No patience. The problem is that on my practice samples the white bottom thread is showing through the top of my design. I added a pic, so I hope it makes sense. I have played with the tension, bought good thick thread, and bought a denim needle to use with it. What else can I check or what am I doing wrong.
Is there a special technique for sewing on paper?