Sagging awning roof

We have an Isabella awning on a 570 van so it has 5 roof poles. 

Whenever it rains we get water pooling on the roof between the centre roof pole and the next one bot the rear of the van. I am guessing that at some point in its previous life the roof has been stretched by water.

any ideas as to how to stop it as getting up all night to push water off with a brush is not my idea of a holiday ;-)