Housemom,
I cannot advise you on when to replace sand in a sand filter since I do not own one but there are plenty of other threads that have covered this topic before. Unless you see a lot of sand in the pool or in your backwash, you probably have not lost much of it.
However, there could be other reasons why your pressure is low on the filter. This usually means that the suction is high on the other end of the pump. Things to look for are valves in the wrong setting, skimmer flaps shut off, pump basket full/blocked, or even some blockage in the suction lines.
Also, what kind of drop in pressure are you taking about? From what to what in PSI?
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