A PStone's field can overlap other fields, but it cannot overlap another PStone itself. If you see a message saying "You cannot place this here" or something to that degree, it usually means that there is another stone nearby that the pstone you would've place would overlap the nearby stone itself.
It is also important to note that this affects PStones placed by other players as well. If one field overlaps another player's field, there can be some block placement/ destruction conflicts.
This is why the placement of Protection Stones is important.
