Looking at https://jcenter.bintray.com/io/grpc/, I see:

Have 1.21.0:
all, api, auth, context, core, netty, netty-shaded, okhttp, protobuf-lite, protobuf-nano, protobuf, services, stub, testing, protoc-gen-grpc-java

Missing 1.21.0:
alts, android, benchmarks, bom, grpclb, interop-testing

I'm going to give it another day. If they don't show up I'll file yet another issue with JCenter.