I agree about the needle size, when I'm working normally I use a 16 (on longarm) when there is flannel or lots of thick seams I may use an 18.
It doesn't look like the center of the triangles would need a bigger needle. so that stumps me.
But I wonder if your feed dogs are interfering somehow, fmq, do you drop your feed dogs
straight stitching, you use your feed dogs
check out the dogs... are they up and down at the right time.