I feel like this sometimes when my abdomen muscles tighten from stress and I only breathe into my chest. I end up taking shorter, more shallow breaths and then wonder why I'm not getting all the air.
If I'm having a hard time doing this, like when I'm standing up, I lay down flat on the floor and breathe in deep and slow. Make sure your stomach rises.
I apologize if you've already tried this. I have no experience with asthma. ^^;