One of the most central themes in Lord of the Rings: TheTwo Towers is the incarceration of King Theoden, into a weakened and feeble state where he is easily manipulated by the duplicitous Grima Wormtongue, and easily controlled by the evil powers of Saruman. It is a very clever tactic of the white wizard to control the kingdom of Rohan, which is on the bordering land to his tower in Isengard, which furthers his reach and his attempt to find the ring. Whilst Theoden is trapped in this terrible malady, Saruman’s Uruk-hai destroy villages across Rohan, stealing food, weapons and supplies, as well as scouting for the dark object that would give Saruman the ultimate dominion he desires.
This is also a clever part of a much larger plan of Sauron himself, for his aim is to let Saruman get the ring, and watch the kingdoms of Isengard, Rohan and Gondor destroy one another in their own folly. He is aware of Saruman’s plan to obtain the ring and control Gondor, but this all suits him and his bigger picture. He knows that even if Saruman did get a hold of the ring, it would still find it’s way back to Sauron anyway. After all, the ring ‘has no other master.’ But fans have always questioned why the men of Rohan don’t seem to notice the spell that their king is under, despite it being so obvious to the surrounding nations.
There are only three people within the kingdom who seem to have an inkling that all is not as it should be with Theoden. These are Grima himself, who filled the king's mind with poison in the first place; and Eomer and Eowyn, the king's nephew and niece. Even though the two can tell that something is wrong, they have no idea what, and no idea how to stop it. That is, until Eomer seems to figure out that Grima is involved and confront him.
It is at this point that Eomer realizes the slimy servant has been in cahoots with Saruman, and that he wants to claim Eowyn as his reward for betraying his king and his kingdom. Grima has Eomer banished from the kingdom with a decree from Theoden, and that leaves Eowyn as the only one aware of what is going on. When the king's son Theodred dies, she pleads with her uncle: "Your son, he is dead, my lord. Will you not go to him?" it is clear to her that her uncle is sick, and that he is not acting in the way that the strong and noble man who raised her would, but she is helpless to stop it.
When Gandalf, Aragorn, and Gimli arrive in Rohan, the guards refuse to let them in the gates without surrendering their weapons, on the order of Grima Wormtongue. It is markedly odd that they are taking orders from Grima, even if he is the advisor to the king. It seems that none of them suspect Wormtongue, or have even registered the terrible torment that Theoden is under at his servant's hands. When Gandalf eventually makes his way into the hall and banishes Saruman’s cruel magic from Theoden’s mind, the transformation is clear to see as Theoden goes from an old, withered, nonsensical man to his young, strong, assertive self. From the audience's point of view, it's remarkable, and it seems impossible that the men of the kingdom wouldn’t have noticed his premature aging and sudden weakness before this point.
However, it is important to remember that the transformation the audience sees takes place in a matter of moments, with Theoden suddenly reverting to his former glory. On the other hand, the process of Grima making him old and withered and poisoned in the first place was months in the making. Neither Grima nor Saruman are stupid. Both are incredibly cunning and skilled in manipulation, especially Saruman with his power to enchant people and make them do his bidding with just his devious voice. Therefore, they carried out their plan painstakingly slowly, bit by bit, so that none of the guards of Rohan would be suspicious. By the time they realized that the king was dreadfully unwell, and no longer capable of ruling the kingdom properly, Saruman’s grip already held him, and the damage was done.
Only Eowyn and Eomer noticed the gradual change that came over the person they loved, because they were the two who spent the most time with him. That is why, when Gandlaf, Aragorn, and Legolas are trying to free him from his torment, Aragorn still has to restrain Eowyn from stopping them, because although she knows her uncle is in trouble, she doesn’t yet understand that magic is involved, or that Gandalf is the only one who can help him.