Thanks for the replies,
The resin is about 4 months old,but when it was delivered the bag was unsealed.
It is a 200 gpd ro-man,not sure the size of resin cannister,but fairly small.
I recently fitted new pre filters and membrane,but this is working ok as the tds is about 8 or 9 after these, but before the resin.
So I presume it is more likely something to do with the resin