IMO the problem is the water makes puddles in the cover and it gives up soon or later.
I am not an expert but DIY solution using piece of waterproof PU nailon to improve the top of your exiting cover maybe?
http://www.ebay.co.uk/itm/Waterproof-4oz...3641.l6368
Motorbike folding cover with some kind of internal support may be the solution?
http://www.ebay.co.uk/itm/Motor-Bike-Fol...FOh4bD0GgA
I am not an expert but DIY solution using piece of waterproof PU nailon to improve the top of your exiting cover maybe?
http://www.ebay.co.uk/itm/Waterproof-4oz...3641.l6368
Motorbike folding cover with some kind of internal support may be the solution?
http://www.ebay.co.uk/itm/Motor-Bike-Fol...FOh4bD0GgA