membranes are rated at 40 PSI, there are lesser membranes on the market that rate their flows at much higher pressures.
That could be why you are getting such a low flow rate. Do you know your pressure? I do not know of a membrane that produces 125 in a single membrane at a reasonable pressure...
Also water temp and water quality play a part in the production rate.
From what you are saying it does sound like a bad membrane, the company can replace the membrane for you. We end up with 1 bad membrane about every 1000 we go through, and we use DOW only. So it does happen.