Why some DROs not oscillate withlout enclosure top covered, but others do?

Some DROs not oscillating without metal plate placed above resonator. As i understand it is not frequency tuning (phase condition) problem, because moving resonator does not start oscillation. Although placing small metal piece above resonator instantly starts oscillation. What stands behind this? Less radiation because magnetic field lines stick to metal plate, or there still radiation exsists and it reflected back to resonator?

either the coupling is insufficient with the cover off, or the phase shift is wrong with the cover off