The volume of emails requesting a lab position received by most professors in my field is large and unceasing. This means that you have at most a few moments of attention from the prof who is skim reading your email. By far the majority of emails read along the lines of "my name is X and I would like to do a PhD in your lab", usually followed by "I have graduated from [insert foreign institution] and would would value the opportunity to perform my PhD in your lab". There is usually no information that describes why the work your lab undertakes is an important part of the decision process that lead the applicant to email the prof. Nor is there a description of what the applicant brings in the way of unique ideas/approaches that will forward the work the prof performs.
Cold emails from excellent candidates will always have to compete with those from less exceptional candidates, and are out numbered by the latter by at least 20:1. Spend the time to personalise the email. Suggest approaches/experimental plans that will address areas of interest to the prof. If possible, go to scientific meetings and initiate a face to face contact that will notify the prof for your intent. Maybe try writing a traditional "snail mail" letter (you know, written on paper and sent by post) - this at least will force the recipient to spend a moment or two longer on the correspondence and may provide a novelty factor that aids in distinguishing you from the rest.
