Proofreading Is Not Reading
Not everyone is a proofreader. As a matter of fact, very few people are adept at what may seem a simple task. To be a successful proofreader, you cannot simply read the content. You must look at each and everyone word and each and every letter of those words. You must be able to predict where errors will occur and look specifically for those situations. And never assume that a word or name is spelled correctly. Look at every word carefully and spell it out letter by letter, if necessary.
Eventually, you will be able to predict which words in your particular field are typically spelled incorrectly and will be able to automatically look for those words. Then reread the text to see if it makes sense in the context in which it is written. Check all similar items for font size and type. For example, if a particular heading appears on numerous pages, then look at all headings at the same time to compare fonts instead of looking at each heading as it appears. This makes it much easier to catch inconsistencies.
Remember, you are not reading the text. You are looking at every word by itself and then in its context in the overall document.