I've noticed that even in San Francisco's Mission District, I can almost never get a Postmate to accept my order. Could be high demand, or could be no supply. My guess is the latter.
I've noticed the same. It's been striking in the last few months. There used to be plenty of availability, but recently, the service is almost unusable due to lack of supply. It makes me wonder if things are much more seriously wrong than they are admitting.
Same in Seattle. After 10 minutes of attempts I always give up and use someone else