If your sediment and/or carbon block filters are blocked then that will reduce water production.
You are supposed to replace these regularily. I replace mine every 3 months, but some do it every 6 months.
However if everything is right, ie 50/50 water to waste ratio, clean prefilters etc., then the problem is probably due to the water getting colder. As the water gets colder, all r/o's produce water slower.