ASAIK, on 15.1 we dont have old dhcp, its the new jdhcp and your configuration is also for new jdhcp model.
Can you run below commmand to confirm if JDHCP is running?
root# run show system processes extensive | match dhcp
1281 root 1 96 0 50280K 12060K select 19:38 0.00% jdhcpd
And if you see jdhcp running please use "restart dhcp-service " to refersh the process.