Just from experience I would increase Muti Media backwashing frequency if you are running 20 hours per day.
Is the raw water source chlorinated?
With the antiscalent I would recommend having a projection run on the water analysis to make sure that the dosage is correct.
I could do that for you on Monday it would only take a few mins. Just shoot me the numbers.
Could it be bio fouling? Do you have the capability of cleaning the membranes on site? TDS Loss is.always hard to get back where flow is much easier to deal with.
What brand of membranes are you using Filmtec or another brand?
Do you have sample ports on your pressure vessels? They make isolation a snap. If you do have to pull the elements check the U cup seal they sometimes roll off. When you reinstall the elements use glycerin on the seal and the membrane outer shell and they will fall right in .