There is something missing in your story. 19 PSI is a backpressure developed by the filter element. If you had that backpressure agaist clean filter and have the same 19 PSI now, your flow volume should be pretty much the same. This is how you determine the cleaning cycle.
How you determined the slower flow? By the suction in the skimmer or turbulence by the fresh water inlets? Remember one thing: if you had a rain, or added couple of inches in water level, it will not show the same action by inlets or the skimmer.
Your skimmer is the most effective, when the water level is only about 2 inches above the intake ledge. The higher water level, the more debris will just float by.