After making hundreds of coffees in the Aeropress, the rubber plunger end can sometimes dry out a bit, or become compressed or worn. This can allow some air leakage and a not-so-airtight seal. You can just pry off the old plunger end and pop a new one on instead of throwing away your entire Aeropress!
Some Aeropress owners notice a sticky residue on the plunger end of their Aeropress. This sticky excretion is a combination of coffee oils and mineral oils, and tends to occur when an Aeropress has been sitting unused for an extended period of time (especially if the Aeropress is stored in a warm place). While this sticky material can be difficult to wash off, the oils are harmless as is required for the food grade materials used in the Aeropress.