I've been querying for several months now without much success. In fact, I've had no success. Not a single request for more pages. It's terribly disheartening because I've had my MS thoroughly edited by a professional editor (which cost quite a bit) and spent several months honing my query.
Without any feedback from the agents I'm querying - other than the occasional form letter - I'm not sure if it's the query that isn't compelling enough or the pages.
Do you have any advice on how I could find out?
I feel your pain. Querying is soul destroying, especially when you're not getting any helpful feedback. It's hard to know if it's the query or the pages that aren't working, or if you're writing in a genre or category that agents know is on the way out.
The best way to figure it out in the first instance is to change up your query and see if that makes a difference. If you suddenly see an upswing in the number of requests you get, you'll know it's the query making the difference.
If you change the query and still get no change in the number of requests, it's time to take a good look at your manuscript. One of the things I find frequently in the manuscripts I critique is that the book starts in the wrong place. The writing might be lovely, but unless the plot kicks off within the first few pages, you're probably starting in the wrong place. Agents usually only give a book a few pages to wow them, so if you're opening isn't compelling, you're not going to get any requests.
There are a number of places you can get help with your query package, and it's well worth taking advantage of any assistance you can get. When you're as close to your book as you have to be to write it, it's often difficult to see where your query or opening pages are letting you down. If you have a critique group, get them to look over your query materials and give you feedback. Authors and agents sometimes offer critiques of these as contest prizes or auction them off for charity. Enter these and you might get something very valuable in return.
There are also forums you can join where other writers will critique your query. The SCBWI boards is one place. And I believe there is a socializing section on Query Tracker that allows you to trade queries with others. I haven't used this in a few years though... And then there are networking opportunities through online conferences like Write On Con where fellow writers will comment and critique your query.
For the very brave, there's also the infamous Query Shark, where agent Janet Reid will set you straight about what works and what doesn't work in your query. Even if you're not willing to let her shred your work, you will learn an enormous amount by just reading through all the queries she has already eviscerated and helped to re-build.
Hopefully this will help you hone that query package, and start getting requests.