When you try to heal a player, and he only moves 1 cm or less (accurs also if he only turns around) the heal action will not be completed - the heal animation runs normally, but the player will not be healed.

When try to heal in proned position, while hitting the action the healer gets moved closer to the to be healed person and thus the to-be healed person gets moved away from the healer.

Because of the problem of slightly moving to-be healed player, the action will not be completed.