My payment gateway on Membership isn't working

I installed Membership and signed up with Authorize.net ARB as payment gateway. All my settings seem to be right but it keeps rejecting card payments.
I called their help centre and they tested it. They say the request doesn't reach them. Everything else works fine but the payment part.
Is there any way I can fix this?
The website I installed it on is http://etpenterprises.com