Resume help for stay-at-home mom returning to work?

I am a 36 y.o. and have been a SAHM of 2 boys for 4 years. I have been separated for 6 months (after being married 12 years) and am having difficulty finding a job. Before we made the decision for me to stay home, I worked for a big box home improvement store as head cashier for 4 years. Before that, I worked as an Admin. Asst. for a financial co. for 2 years. And prior to that, I was a Business Manager for an internationally known cosmetics line for 3 years. I was working before that but most places don't require more than 10 years of work experience. During my time as a SAHM, I volunteered some but due to financial difficulties we had only 1 car for a long time. I have been sending out my resume but I keep receiving "We're sorry..." letters even though I have the experience listed in the requirements. I have my time as a SAHM listed so employers don't wonder what I've been doing. Could that be affecting my chances at a job? I've done research recently and have seen that Functional Resume formats are the way to go when you have a gap in work history. Do hiring managers respond to this type of resume format? I am an intelligent person with some college under my belt. I'm mature, dependable and friendly. Why is it so hard for me to find a job? I'd like to hear from people that are in the position of hiring prospective employees or other moms that had to return to work. Thanks!