You don’t say what font(s) you are specifying – are you sure that you have the correct font face and size? If this is a Windows machine and the font size is 10 pts or smaller, try reducing the specified size by .25 pts (so 8 pt becomes 7.75 pt).
Please also consider that sometimes, even when you’re dealing with text, plain old image capture is the best approach. The TIG was not intended to replace image capture, but rather to provide a mechanism for dealing with situations where image capture was difficult or the sheer number of images that would be required (as in data-driven testing) would be prohibitively large.