Just keep working on it. Everyone has to do it a little differently because everyone's mouth is shaped a little differently. Try to "purr" without a call in your mouth (airflow and vibrate your throat.) Once you can do that pretty consistently, try it with the call. Play around with different amounts of air and tongue pressure until you find one that works for you.
Try putting the call as far back in your mouth as you can, without gagging...then growl like a dog, as you go it'll start making sense, also try not too apply too much pressure with your tongue, it'll start to squeal and thatll do ya no good.
The simplest way I knew was to use the same action with your throat like you were hocking flem. Purring was by far the easiest call for me to make with a diaphram. But I got my tonsils taken out last year and since then I can't do it all anymore. Now I have to find a new method.