I’ve killed about 15 or more dragons and now I no longer receive their souls. Why is this happening?

If you are talking about dragons at the word walls around the world (usually high up in the mountains), those dragons respawn, but do not respawn with a soul. You can only get a soul from any particular dragon once (though the ones I just mentioned are the only ones that respawn, so...)

Thanks for the info!

Does anyone know how many dragons there are? I've collected.... hmm... 37 souls I think, have to look it up.

I *think* the "normal" dragons are infinite. The word-wall dragons seem to always respawn but you get no soul from them. I found (on the only toon who got into that) that some other dragons respawned but didn't provide souls - not sure what if any defining factor they had. That toon I deleted at level 42 or something, having done the MQ. She had unlocked all but 3 or 4 shouts and had about 60 souls in "inventory".

This is how I understand it, too. I have/had waayyy more than 37 on my first character.