What if I can't find an example of where a theory could be applied in the exam texts?

Before analysing a text, it's always a good idea to look at the contextual information that you have been given and come up with some theories which are likely to be relevant (e.g. if it's a transcript, Giles' Accommodation Theory may be applicable). So if you can't find an example, look back over the contextual information to remind yourself and then look at the text again specifically for features that the theories you identified could be applied to. If you have found something that contradicts a theory, then don't be afraid to mention this and also explain why it is that the theory is contradicted, for example changes in society/culture may mean that it is not as applicable (a good one for criticism is Lakoff's Deficit Approach which was devised in the 70s and attitudes to gender have changed since). Remember that you aren't expected to apply every single theory possible, but you should mention a couple when explaining the appearance of features in the text.