A couple of tips or questions for you.
Did you starch and press fabrics before cutting the triangles? I like a good starch and press first.
I like to cut a smidge paste the line on the ruler. Make sure that "the line" covers the fabric and then cut accordingly. I found that sometimes I would cut short and it was due to "the line" placement.
Agree to cut, stitch and oversized triangle and then trim up as needed.
For multiple triangles try the thangles papers.
I also like to mark seam lines on pieces until I get a feel for the piecing process.
What thread are you using for piecing? There is a difference in thickness between brands, etc.
Can you move your needle one click over to compensate if needed?
Best wishes on your project.