You might want to review the helper statistics and configure the trace options recommended in this troubleshooting kb to confirm that the dhcp request is received by the port and actually forwarded.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB11020
If the forwarding is working, then start down the list of this general kb beginning at step 7
https://kb.juniper.net/InfoCenter/index?page=content&id=KB23335