Triangle issues
 

OK, I'm making an 8 point star complete with y-seams. Now you'd think the y-seams would be the thing
I was having trouble with but no, it's the triangles. I cut the the correct size and they fit the template, the seams are correct as I drew seam lines to follow. When I put the triangles on they're too small/short. Everything has gone together perfectly until now. Even the y-seams we're easy.

I just posted a question about the same thing. Go to "y seams", and watch the video that was posted for me. It's really good and may be the answer to your problem.

LOL, I'm the one who posted the video. Doing it her way makes it really easy. I'm starting to think there is a misprint in the pattern I'm using and the triangles need to be taller.

Did you cut the triangles from the same size squares as you used in the corners?

If so, they would be short. The triangles need to be cut (square size +3/8 inches)

Example:
If the corner squares are cut 2-1/2 inches, then if you cut squares diagonally for the triangles, those squares need to be 2-7/8 inches.

Each part has its own template and that was how I cut each piece. The corner squares are 3 1/2" unfinished, the triangles are 4 7/8 across the bottom or long side and 2 5/8 from the tip to the bottom or long side. The triangles just seem too short. They do fit from side to side though.

With those calcs the triangles are the wrong size. Its a good thing I bought extra fabric. I'm recutting the triangles so I hope they work this time.

What kind of 8-pointed star are you making? Is it (what I know as) the LeMoyne star where you use the 45-135-45-135 degrees diamond and all four sides of the diamond are the same length?

There is another 8-pointed star that uses HST units to create it.

So - if you are making what I call a LeMoyne star, maybe the attached diagram might help.