Howls are trickier for me too. I have to position the diaphragm further back on my tongue and apply more tongue pressure and more air than the distress call. Try making some loud barks. Just apply a bunch of pressure and air. Once you get that, you can feel what it takes for tongue pressure and position. Took me a while to get the sounds I wanted from a diaphragm call, but worth the effort. Definitely my most versatile calls. Practice will make it happen for you.
Just reread your question. Since you are already getting a howl, just try to keep your tongue pressure constant. You'll get it with a little time.