You're being fobbed off, if nothing has changed with your water pressure, tap tds and so on then there's no reason why you shouldn't be getting the same results or close, even with a 20% difference it wouldn't go from 5 to 40.
I would run it on flush for a few hours and see if that makes a difference, if not you have been sold a dud, also try a differnt set of pre filters just to be sure that they arent the problem.
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