Problem upgrade sites servers to 7.5

Hello to a new problem, I have problems to update site servers in my environment, I use the policy update SMA (site-servers only), and the agent is reflected politics, but the status is always in "downloading" and makes many retries download but never downloads, if I try to update any agent that site is the same, try downloading the package manually and install it, when I do that the agent is damaged and I can not uninstall or install the site server again, once there is no longer the agents site server starts updating directly from NS, which will be happening? that I can not update the site server? and that if the package is installed, the agent is damaged?, I'm migrating from version 7.1 to version 7.5, and there are subnets and sites created, please help