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).
18When you see the trouble they had finding the rebels base in V, you can imagine how hard it would be for them to find a single person (one of the most powerful Jedi at that) Oct 12, 2011 at 14:21
55@LoïcWolff: Exactly. Not to mention, even if someone DID find Yoda on Dagobah, Yoda would wave his hand and say, "Find me, you did not. Devoid of intelligent life, this planet is."– JeffOct 12, 2011 at 14:28
25Approve of this, I do.– user2900Oct 12, 2011 at 20:05
4@Jeff's comment is spot on: you need to search an entirely galaxy using only strong-willed people who can resist mind tricks. There just aren't enough people.– TynamJan 3, 2012 at 13:49
11@Jeff, but when the trooper reported back with "Find him, I did not. Devoid of intelligent life, that planet is" it would have been a huge tip-off. Mar 9, 2016 at 18:47