I found some additional information:
See
https://www.cisco.com/en/US/docs/security/asa/asa70/configuration/guide/vpnadd.html#wp999516Configuring DHCP Addressing
To use DHCP to assign addresses for VPN clients, you must first configure a DHCP server and the range of IP addresses that the DHCP server can use. Then you define the DHCP server on a tunnel group basis. Optionally, you can also define a DHCP network scope in the group policy associated with the tunnel group or username. This is either an IP network number or IP Address that identifies to the DHCP server which pool of IP addresses to use.
==> I think you have to define the scope on the group-policy
If this does not work, you can go troubleshooting:
What type of DHCP server is it? Can you check the logging there?
On the ASA you can do a capture of bootp packets to see what's happening.
There is an option in gui to initiate it.