Diarrhea Woes

My mare has had loose stool and intermittent diarrhea for about 10 days. She had something similar a few years and was diagnosed with having some sand in her gut so I called the vet to ask for advice. She recommended the same treatment as last time and wasn't overly concerned by the symptoms my mare had. (This was all done over the phone)

I've been giving her 5 oz. psyllium pellets, 3 tsp probiotic powder, and 1 tbsp loose salt every day, for the past 8/9 days. I've seen some improvement as now about 50% of her manure piles are fine but the rest is loose. My concern is it's been over a week and she still isn't fully better.

She had a FEC done in April which was low and was dewormed with Ivermectin. She hasn't been dewormed since then. She is on a dry lot and gets alfalfa/timothy hay and a ration balancer. She isn't showing any other symptoms (no fever, not lethargic, no colic signs, good appetite, drinking fine, etc.) but I'm worried about how slow going improvement seems to be. I'm thinking about getting the vet out come first thing Monday. Is it normal for it to take this long to resolve if it is sand? Any ideas as to what else it could be?