1. The matters that can obligate repetition of the before blessing:

There are various reasons for why a before blessing that was said over food may need to be repeated, either over that same food or over a different food of that same blessing.

1. Speaking and other intervals: Speaking, and the performance of actions and other intervals are not considered a Hefsek which requires a blessing to be repeated unless they took place between the blessing and initial eating as explained in length in chapter 7 Halacha 3-4, and as will be explained in Halacha 7-8. However, once one has eaten and swallowed the initial food of the blessing, simply speaking and doing other actions between bites, do not constitute an interval to require blessing repetition, unless one of the following has taken place:
2. Hesech Hadas: There are various different types of Hesech Hadas, or removal of the mind that may require blessing repetition. 1) A resolve to not eat any more of the food may require one to repeat the before blessing if one reconsiders. This is known as Nimlach. [Halacha 2] Likewise, 2) the recital of an after blessing is considered Hesech Hadas which requires blessing repetition. [Halacha 4-5] 3) Sleeping [Halacha 7] 4) prayer [Halacha 8], 5) Speech and other actions [Halachas 9-10] 6) using the bathroom [Halacha 9], are all under discussion as to whether they are considered a removal of the mind.
3. Shinuiy Makom: Changing locations, such as going from one room to another, or inside to out and vice a versa may require one to repeat the before blessing. Due to the length of this topic, a full chapter has been dedicated to it and will be discussed in Chapter 20!
4. Ikkul: If enough time has passed for one to digest the food eaten, then he may be required to repeat the before blessing if he decides to further eat. This matter is relevant to both the before and after blessing, and will be discussed in  chapter 21 which is the chapter dealing with the laws of after blessings.