A deployment task would not be used for new agent installs but you might try pushing the agent out this way:
Go to System Tree view.
Under System Tree Actions - Select 'New Systems'
Under How to add systems, either of the 'Push' radio boxes have a selection at the bottom of the screen called 'Push Agent Using:' where you can select an agent handler.
Hopefully that would take care of it.
Agent deployment tasks are really for agent upgrades. there's some notes on this in the on-line help for the agent if you need.
In that case a deployment task is probably not required if you only need to refresh the sitelist.
Agents will pick up a new sitelist automatically when they call into ePO, providing ePO says there is a newer one available.
Take a look at Configuration - Agent Handlers.
You might just need to make a new handler assignment rule for the machines you want to have use a given Agent Handler.