Neighbors dissapears

Hello! I have the following problem: several servers connected via openvpn tap tunnels if I set "type broadcast;" on tap interfaces - everything is ok but I'd like to filter neighbors and set type pointomultipoint; strict nonbroadcast yes; neighbors { x.x.x.x; } at initial moment - everything is ok, "show ospf neighbors" shows all defined neighbors in state "full/ptp", but after some time part of neighbors dissapears!!! tunnel is ok, I can ping neighbors ip, tcpdump show ospf hello packages. only possibile solution to return neighbors back is to call "restart all" or simply restart bird.. any ideas how to make neighbors to be all the time?