There could be different issues causing the problem. Check the corners to make sure they are at a true 45 degree angle. Do you pin or clip the binding to the edges to hold in place? I do sometimes and not at other times.
Try folding over the binding about 6 to 8 inches down the length of quilt. Make sure it is on the quilt where you want it. Hold it as it goes through the WF. Don't stretch or pull on either quilt or binding. It will cause curvy bindings. A little tug along the way should be fine.
Do you press the binding before attaching to the quilt? I don't as it creates a false fold line.
Here is a post about the topic.
Ironing Binding Strips *** Why Not?
Hope that helps!