Filter your syrup at bottling temperature (180o to 195oF). Otherwise, if the syrup is too hot, new sugar sand will generate during and after filtering.
Inspect your filters to make sure there are no tears or holes.
If you are using Flat Filters, put a small mark in the corner of each filter to indicate which side is UP. Then, whenever you use the filters you can make sure they are facing the same way. Otherwise, if there were still particles inside the filters from a previous session, and later you reuse the filters upside-down, those particles could dislodge and fall into the filtered syrup.
If you are using Flat Filters, make sure your stack of filters is creating a bowl shape in the filter tray. Otherwise, syrup could sneak around and bypass your filters. If you are having trouble maintaining the bowl shape, you can use clothes pins or binder clips to hold the filters in place.