jennanneray,
I don't know anything about the AG Mason made Improved New Wilson machines. I looked it up in ISMACS needle list:
{
http://www.ismacs.net/needle_and_sha...eedle-list.pdf } and it shows the Mason New Wilson uses a 20x1 needle. So a 15x1 is too short and there is your problem.
Here is one source of 20x1 needles: {
http://www.sewingshop.com/singerindu...dles-20x1.aspx }
Don't throw your old needle away. If it was the correct one you can sometimes seat a 15x1 out to the overall same length as the 20x1 and have it work. Not always. The needle clamp on some machines won't clamp the shorter needles.
Joe