The most common occurrence is that something other than text is within the picture. We can only recognise standard text and recommend that you crop anything else that isn't. When the image is processed we convert it to black and white, if you spot anything that isn't text in the converted image then please try again.
You should also try and have the highest quality screenshot possible, zoom into text before taking a screenshot for example whilst also making it as big as possible.
Do keep in mind that you can edit the text before saving it so if there is only a few mistakes then you can correct these yourself.
Please take a look at the Camera tips notebook that is created with your account as well as the tips section on this support site.
If you feel that your text should be recognised then please contact support and attach the image you are trying to convert. We are always looking at ways to improve our recognition and the more feedback we get the better it'll be.