PPTP ping erorr

hi

i have impalement a VPN-PPTP server to provide serves to my customer

i have windows Clint and mikrotik 750 Clint here is my network
vpn.jpg
the main problem is

i can ping the windows client from the server but i canot ping the mikrotik client
and
the windows client can ping the server and the all windows Clint but canot ping the mikrotik client
and
i can ping the server from the mikrotik client

the main idea is i need to ping the mikrotik Clint from the server and from the the ather clients

i have disable the firewall in mikrotik