Always good to at least tell us what version of the software you are using and what types of files you are translating. This can help guide the response to make it more useful for you.
Generally when this happens it's because the file has not been prepared correctly for translation. Depending on the filetype there are various ways to handle embedded content, which is what we are probably talking about if the tags are showing as text.
If you are unable to affect the prepared file because this was don't by others and you only have the SDLXLIFF files for translation then you may be able to add tags afterwards using the Cleanup Task app from the SDL AppStore: