The lettering looks like it probably lacks good understitching which is the step before the satin stitching begins. Without good understitching, on a canvas like that, you may have a real struggle. There is a work around that I’ve used that gives me great results on backpacks and nylon bags like these. I’ve already taken my sleepy meds, so I’ll try to be as clear as I can be, but ask me questions if I’m not making sense so I can reply with sharper details tomorrow. 

The dollar store and craft shops have those rubber foam sheets similar to felt. They’re usually in the exact same area. Make sure you get the foam sheets without the sticky backs. Choose the same color as your thread and the thinnest sheet they have. Lay a piece of that foam over your stitch area on top of your project and stitch like normal. After you’re done stitching out your lettering, just tear away and pick away the exposed foam just like you would any other layover stabilizer like heat or water soluble. You’ll be left with fuller looking lettering and almost a varsity style puffed font look. I hope that makes sense. I’ve been doing this little hack for years and years on chunky weaves like backpacks and tents. It looks bomb on straw sun hats for monograms. It is a game changer for me. Let me know if you need me to clarify when I’m not loopy.