Site to site IPsec VPN tunnel not coming up in box but works fine in forticlient

I have two links, one ILL and other broadband.Forticlient VPN is connecting properly through broadband link but the same is failing to getting up while connecting  site to site VPN in firewall(box). There is no issue in tunnel configuration because it is getting up while connecting through other link(ILL).What may be the possible issues?Please help