Check your pool hose pipes for blockage or crimps in the lines. The pressure will drop if something is prohibiting the water from circulating through the filter, resulting in low pressure. The reason why the pressure drops is that there is no activity inside the filter itself to build pressure. When a filter gets dirty, the internal workings of the filter increase the pressure because too much dirt and debris is built up inside. Check the pump basket and skimmer baskets for large pieces of debris --- most commonly, leaves will really decrease pressure.
Something could be preventing the proper water flow to the filter as a result of a blockage. The low pressure can cause the sand to loosen in the filter. It should be backwashed and rinsed so the sand can become recompacted and ready for normal operation. Usually a filter needs backwashing when the pressure gets too high, but occasionally the pressure will drop due to improper water circulation, and backwashing can sometimes restore the proper filter efficiency immediately, especially after a blockage is removed from either the skimmer or the hose. Also make sure the backwash valve is set back to the "Filter" setting.
Water Level Too Low
The water levels will lower during the season as more people move around or jump into the pool and the water spills out. The water level must always be above the skimmer to provide adequate water space for the filter to siphon the water through the skimmer basket. Routinely check the water levels and raise them as necessary.
Bad Pump or Gauge
If all else fails, the pump motor may not be strong enough to provide enough suction power to the filter. As a result, the pressure will drop because the water velocity through the filter is severely decreased, which will cause a drop in the pressure gauge. The gauge could also be the source of the problem, although this issue is not as common. Make sure the gauge rests on "0" when the filter is off to ensure it's working properly.