Multiple Site VPN Issue with subnets

Greetings,
I ve been searching around for an answer to this problem for the past 2 days but i cant seem to figure it out. Ok heres the deal :

Site A: 192.168.29.0/24 (mikrotik gateway at 192.168.29.1)
Site B: 192.168.5.0/24 (mikrotik gateway at 192.168.5.1)
Site C: 172.16.28.0/24 (cloud router OS)

I have created a pptp based vpn . In more details both sites have local address : 172.16.28.1 while Site A has remote ip address : 172.16.28.8 and Site B has 172.16.28.7 . I have setup all routing at Site C and I can ping everything from everything (computers behind all routers can ping each other ) , ssh works fine but when i try to loging to mikrotik at Site B (address : 192.168.5.1) FROM Site A , the winbox stucks at “Downloading Plugins”… There are a few ip phones as well with a web interface at Site B and while i can ping them fine from Site A i cannot login when i type their ip address on my browser. Its driving me nuts! I would really appreciate some help pointing me to the right direction .

Thanks in advance
Savvas

Solved : it was an MTU issue. Reducing the MTU from the default 1450 on my pptp server down to 1440 fixed everything.