Hello,
please, check the settings of JoomSEF plugin.
It should be set to "override" Joomla default meta. Also make sure it is published.
If you still see Joomla global META overriding those you have set in JoomSEF, try experiementing with JoomSEF plugin ordering (generally the recomendation is that it is ordered first, but the might be other plugin present that will override the META back to defaults) and/or disabling some other plugins that might be interfering.
Also please make sure you are using last versions of everything, including Joomla.