I had a very similar problem to this whilst learning this move. Its a simple fix. When you're riffling the cards together, make sure the riffle is done VERY loosely, by this i mean do it very gently and get some air between the cards. Dont grip the cards really tight and riffle them together really tight.It can even be really sloppy if necessary. With some air between the cards there will be very little resistance. Its difficult to explain, but if you watch Jason Englands 1 to 1 and the way he riffles the cards together, you'll see what i mean!
Hope this helps, and good luck
Tommy