thank you for the answer. I checked and the scan settings properties look ok.
I think my problem is that only parts of the scope are included (and other are excluded) from the Address Pool. It looks like IPAM only scans the IPs that can really be assigned by the DHCP server. The scope in question is used for servers. Most of those only use static IPs and a few use DHCP.
I manually added a range of IPs, that are not not included in the address pool, to the corresponding subnet and those IPs got scanned without problem.