O-ring problem

Hello, this is a new member here:) 

We sold a RO system with output ranging are 600 GPD to a customer, after receipt this equipment, our client complained the 4 inches 300 PSI FRP membrane housing have a problem, its O-ring is slip on, with no gravel. It slipped out every time the end cap is taken out.

But it is our design, according to our technical experiences, no O-ring groove, it can make us to take down the end plate and RO membrane easily, then, the O- ring can slip out automatically. When the RO membrane was installed in, and the raw water has entered into the vessel, the O-ring can not be slipped on, because it will be supported by the water pressure.

Could you tell me your attitude? Thank you!

Jim