Recently I have noticed that when I alter a paragraph or character style that the text won't change to the changes I performed.
It doesn't matter whether it's just the preview mode or when I clicked "OK"- no changes will be performed whether it be typeface or size.
Does anyone know what can be wrong?
Given the lack of information, I can only guess.
Are there manual overrides on those styles? Check the styles for + next to them with the text selected.
My bet is that you haven't got a clear understanding of the difference between character styles and paragraph styles, and that you are using the former incorrectly.
Paragraph styles include basic font formatting information that is applied to all text in a paragraph UNLESS that text also has either a character style or a local format override (select the text and change something) applied. The formatting heirarchy works with paragraph style at the bottom, character styles trump formatting applied in the paragraph style, and local formatting trums everything else.
Select the text in question. Look in the the Paragraph Style field in the Control panel, or open the Paragraph Styles panel to see what is applied. If there is a plus sign next to the name local formatting overrides have been applied (hold the Alt or Opt key and click the style name to remove them), but character styles are not considered overrides, so if there is no plus sign, check the Character Style field in the control panel or ope the Character Styles panel to check if any character styles are also applied.