This sounds like you are ending up with something that is a bit too big. I've run into this problem and it was when I'm not lining up my triangles quite right - IE I'm sliding them a tad too far up and that makes the size just a bit wider than it should be. Often I'll make them a tad bigger and then trim down, making sure I've got the seams lining up where they should be, so that means cutting all four sides, not lining up one and trimming off it.
I do a lot with my easy angle/companion angle and also the one used to make half rectangles (don't recall the name).