There are two common reasons that highlighting (or underlining, etc.) or text selection may not work.
The first reason is that the document is just a scanned image, and doesn't actually contain any text information. You can usually see that this is the case by zooming in really close on some text. If the text gets blurry or loses sharpness, then it's likely an image. Real text stays sharp and crisp at any zoom level.
The second reason is that Xodo may not be able to handle the font of the document. We have done our best to let Xodo handle various fonts from around the world, but there are just so many fonts out there. Xodo will do its best to find a similar font if the document is using a font that isn't installed on the device, but might not be able to find a good replacement. If this seems to be the case with your document, please send us your document with an explanation of the problem if possible. We may be able to solve your issue for a future release.
In the case where highlighting or text selection doesn't work, there are a few alternatives.
You can use either rectangles or a thick ink with a lower opacity (25 - 50%) as a highlighter.