you are confusing terms here a little - the RO stage uses a membrane (75gpd or 100gpd typically, there are "larger" ones i believe) while the DI stage uses resin - your resin is exhausted, there is likely nothing wrong with your membrane.
Properly maintained, an RO membrane should last nearly forever, while the DI resin will need to be replaced on a regular basis along with your carbon and sediment pre-filters.