Most of the times DHCP Relays does not work because people forgets to enable the DHCP service in the global configuration of the network device:
Before you configure the DHCP relay agent, you should understand the concepts documented in the "DHCP Overview" module.
The Cisco IOS DHCP server and relay agent are enabled by default. You can verify if they have been disabled by checking your configuration file. If they have been disabled, the no service dhcp command will appear in the configuration file. Use the service dhcp command to reenable the functionality if necessary.
The Cisco IOS DHCP relay agent will be enabled on an interface only when the ip helper-address is configured. This command enables the DHCP broadcast to be forwarded to the configured DHCP server.
I do face the same issue
For me, the setup is working properly and suddenly goes off, during the time i can't see any relay packet from SVI to DHCP server. This issue is for only one SVI, all other are working.
I am using 3850 box with Image: cat3k_caa-universalk9 SW ver: 03.03.05SE
I've seen routers with this problem a lot. Two workarounds always worked for me.
Enabling and disabling service dhcp few times.
no service dhcp
also removing the ip helper-address or adding other dhcp servers in your environment.
Hope this helps.