It is possible that your mother was denied another loan due to having the parent PLUS one. When companies consider you for a loan, one of the things they look at is how much you already owe in debt. So depending on the size of the PLUS loan, this could be the reason.
And yes, you can defer your loans while you are attending school. You just need to send a Deferment Request Form to the Department of Education.
Be sure to follow us on Twitter: http://www.twitter.com/finaidforum