Occlusion occurs when all of the drug product that is available has been collected. This is often below 100% for many reasons. The filter may be absorbing drug substance, also the cannulas, syringes, vessels, and shafts can also be guilty of holding on to your drug product. Some may still reside in a non-disintegrating tablet, or undeaggregated material. There is also some variability with content uniformity that must be taken into account. (This is why Q is usually 80% or less.)