Awning - putting up

Hi we are new motorhomers, never having erected an awning before.  We think there is a problem with our new freestanding awning, but the supplier does not agree. One joint at the rear corner on both sides of the frame does not lock home so can easily slip out when we push the awning up, also we have to pull the canvas really hard to get it over the frame  stretching it so far it just seems wrong.  Any comments and advice?

Carol