don't put a lot of pressure on it in your mouth. there should be an air seal between the floppy edge of the call and the roof of your mouth. Air should only pass through in between the reed and your tongue. start with very light pressure. Work at it till you can get a clear note. Increase pressure till you can get a clear high-pitched note. Once you can hold a nice high note, drop your jaw just a bit so the sound breaks down to the lower second part of a yelp. Start real slow so you can distinguish between the high and low notes. Eventually you can do it a lot faster and it will kind of blend together to make a yelp. For clucks, putts, etc, the best way I can describe it is saying "tick" without the vowel... make a "tck" sound with your tongue making sure it passes through the reed. Vary pressure between your tongue and the call to get different tones. Purring involves trilling your tongue and is a bit difficult to get the hang of. I found it a lot easier when I relaxed the tip of my tongue to let it vibrate more freely.
Hope this made some sense, sorry for rambling, and hope it helps a bit. I have only been using a diaphragm for 2-3 months but can yelp, cutt, cluck, and purr.