If the filters arent the issue the membrane also can be. Do you have a flush kit? they work well. If you still have an issue before you go and buy a membrane go to the harware store get a guage and check your pressure.
I have a membrane flush kit, and I religiously flush it every time before I start making RO DI water...I really don't think it's the membrane that's the issue. I changed all three filters pretty recently, it should be the clogging either...but I will take them out individually to check it out...I was dreading about having to spend more money on the equipments :-( uggghhhh