making spikes pointy?

I know this should be a simple fix, and maybe it's already been posted, but I wouldn't know where to look.

How do you get spikes, horns and things, to have pointy tips? I've been using just a basic triangle shape, and every time the tips end up being round nubs. I try pulling the fabric with needles and other things, but it doesn't help much, especially on the type of vinyl I have.

I tried different stitches, such as a blanket stitch, but it still does the same thing, only making the seems be shown on my vinyl.

Is the shape of the spikes? Or am I doing something wrong?