I've been at this now from almost ten years and over that period the ability to drift the ball has alluded me, sometime appearing when practicing and never being able to figure what it was that caused it. I've asked numerous people about it, looking for the definitive answer and it's always been vague or at the other extreme - explained in complex terms using physics. Then last summer after a massive discussion on a forum trying to make sense of the physics one of the blokes said something along the lines of... "You'll always struggle to get the ball to drift even if you are spinning it hard because you bowl with a very vertical arm".
I thought there might be something in the comment and decided at the start of this season when I injured my knee and had to bowl off of a 1 or 2 step approach to the crease, I'd look at trying to bowl more round-arm.
With a little bit of work, I found that I was able to get the arm down a little lower and still maintain what little accuracy I have at the moment. Then low and behold, in only the second session - there it was... very obvious and sometimes 'Big' drift. Not just every now and then, but almost every ball!
I've worked with the round arm approach since then gradually developing it and improving it and have discovered the round arm approach has other attributes. The ball does that amazing thing where it looks as though it's been bowled too full, then suddenly drops like a lead balloon and drifts on its way down and then turns off the wicket viciously. All attributes that I've been looking for, for years!
As the weeks went on, I became aware that the wind direction is a key factor. Without the wind the chances of the ball drifting are limited and possibly even negligible. But with the wind coming from the off-side blowing towards the leg-side, it's virtually guaranteed every ball with the round arm action which for me is an amazing revelation. So if you're not getting the ball to drift I would certainly look to lower your arm and give the ball a rip. You can see the ball move through the sky with side-spin as it goes down the wicket, try and get that side spin going because it helps.