Earthquakes nearby, magma chambers getting too close, ground water leaking into the layer or out of the layer, chemical changes, dissolving of isotopes, or crystallization of incoming isotopes – are all examples of things that could easily happen to a rock sample underground, in only a matter of centuries, not necessarily millennia nor eons of time.
Scientific measurements are not allowed to be based upon the hopes of being extremely lucky.
Radiometric dates are therefore based upon extreme wishful thinking – something very far from the objectivity of true science. This means that they release alpha particles, in the form of alpha radiation, as they go through the decay sequence and become the product element (like Uranium turning into Lead, for example).
An alpha particle is made of two neutrons and two protons stuck together.
Helium can soak through solid rock, but not very quickly.