The first, quite glaring example of this is the death of William, which is caused by the monster. Frankenstein, catching a glimpse of him in the neighbourhood of the crime, immediately thinks he was responsible. Obviously Frankenstein never intended him to go around killing people, so this is the first real example of how Frankenstein retains no power or control over what the monster does.
Another example is rather more interesting, and subtly different. When the monster ominously promises to be with Frankenstein on his wedding night, Frankenstein assumes that the monster will come and kill him then. Although he resolves not to die without a fight, the monster outwits him by killing Elizabeth, his wife, instead. This shows that Frankenstein simply cannot predict what the monster will do, even after meeting and talking with him.
In a more general sense, the indication that Frankenstein has no control or power over the monster is simply that he cannot even bear to be near him. From the very beginning, Frankenstein is totally revolted by his own creation and does not want anything to do with him, except to pursue his revenge on him towards the end of the story. He does not succeed even in this, however.
You could say that Frankenstein never lost control over the monster since he never had that control to start with. The monster never served its purpose (to make Victor proud) because you cannot give purpose to people (and the monster is a man, as he proves it with his unmistakenly human behaviour). Existentialists claim that there are four givens of human existence:
1. Isolation. You are human if you understand that you are alone in this world and that you can never truly and completely know another person (haven't you ever felt that no matter how hard you try you could never explain yourself to others?)
2. Death. We humans are the only species that knows that we live only to die. We defend ourselves by believing in some sort of a sequel, a life after death, but the truth is that death terrifies us no matter what we believe.
3. Freedom. This is a particularly painful aspect of the human condition. We all crave freedom, ability to act as we see fit and if you are stubborn enough you just might get your way, but this means that there is no greater force that unifies us and gives our lives a purpose. If everybody can do what they want then tere is no right answer to the question what we ought or ought not to do.
4. This brings us to the final givens of the human condition - meaninglessness. We are a meaning-seeking creatures who are thrown into a universe that has no meaning, no preordained design, where everyone must construct their own meaning in life.
These are all complex emotions the monster battles with, as any other human does. His behaviour was never conditioned by Victor since he hardly even spent any time with it. Victor gave up his right to influence the monster's behaviour when he left him. If he had been more responsible towards his creation, the things might have turned out differently for the monster (not Victor, he crossed all sorts of lines when he decided to animate the dead, but that is for another topic).