Hello Sam,
First, your credentials set you far ahead of most candidates, so this likely means you are having a communication problem. Namely, how do you communicate your outstanding qualities to a stranger so that they have a strong impression of who you are?
I would guess without knowing more about you that your cover letter is not expressing your unique qualities very well. Writing a fantastic cover letter is essential to a job search. I spent a month crafting mine and scrutinized every word. Unfortunately, how to do this is way outside the scope of what I am prepared to do. But safe to say there has to be something in that cover letter that is so irresistible to the reader that they put down the page and go to the web to try and find out more about you. That's your goal: to be remembered and engaged with in a deeper way.
Next, are you using those irritating online e-submission mechanisms for your package? If so, you are sending it directly to a computer and not to a human being. My advice is to locate the name of a portfolio manager at the firm you are trying to reach as well as the mailing address for that PM. Mail rooms are trying to solve a different problem than the HR department. Mail rooms are simply trying to route traffic, whereas HR is trying to gatekeep. If the PM receives a nice cover letter and CV that is well drafted, expresses enthusiasm and insight for the business, and is compelling in its packaging then they are likely to read it. I can tell you that in my career I received less than five such letters directly sent to me and all of them received phone calls when we were in a hiring round. I can't make promises for people I don't know, but if they are kind, and most on the buyside of the business are kind, they will remember you and some may even be open to further discussions. Also, here is a dirty secret of the business, most HR departments have no idea how to hire for a PM or research analyst job - they are terrible at it. So they are likely to overlook the important distinctions of a candidate.
Next, invite a mentoring relationship with this PM or feedback from them if they aren't into mentoring in your cover letter. Here on the wild world web you have found someone willing to give back to his community by proffering free advice. I can't be the only one : ) If no PMs take you up on this, try approaching head hunters for the asset management industry. Some are willing to offer feedback on cover letters and CVs.
Last, be patient. It took me a year of looking to get 3 offers out of 550 packages sent out. A pal of mine took two years to find work. Are you able to do something else while the process grinds on? For example, work as an analyst at an E&P, minerals, or commodities firm? Usually the buyside recruits from industry.
Best, best wishes for success!
Jason