Been a while since I used a Cisco router as a DHCP server.  Tried this config on an 1811 running IOS 15.1(4)M6:

ip dhcp pool MyPool
   network 192.168.1.0 255.255.255.0
   subnet prefix-length 24
   dns-server 192.168.1.100
   default-router 192.168.1.1

But clients could not pull a DHCP address. Static IPs worked fine.

debug ip dhcp server event would ultimately show this:

Mar 20 15:46:23.441: DHCPD: there is no address pool for 192.168.1.1

Removing the subnet prefix-length statement magically fixed it

DHCPD: there is no address pool