At the end of Revenge of the Sith, the Emperor knows thats Yoda is alive. Why would he not make that the first priority of the Empire? Yoda would be the one Jedi of all the Jedi you would not want alive, so why overlook this?
The galaxy is a big place. Even the Empire can't overturn each stone, shake each bush, and question each person.
The search DID continue, but the Emperor knew better than to think it would succeed: Yoda is ancient, wise, and powerful with the Force. He's one of the few Jedi who escaped Order 66 by dealing with the clone troops instead of not being near them or evading them.
Throwing wave after wave of men into a search would have been fruitless - the only information on his whereabouts would come from which groups never returned.
The Emperor and Vader searched quietly, and Yoda just hid too well (being the only sentient on a planet and living next door to a veritable font of evil power being one of the more effective ways to hide I can imagine).