you haven't mentioned about which version of SharePoint it is. assuming it is in 2013, User profile services are separate application. you can hold more than one User profile service application and connect different User profile services app for each web application. if a user is not resolved in one web app, check the connection of which user profile service app is configured in that web application and check the sync status on the specific UPS.
you can manage the connections in 2013 central admin
CA-> manage web applications -> click your problem web app -> from ribbon choose the "service connections"
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