Cannot scale our redis cluster

we installed redis cluser on GKE. We have 3 node dedicate to redis. Started 12 replicates ( 6 maters, 6 slaves) each with 4 CPU and 8G memory.
the maximum rate i achieves was 20K operations per second (set function, different keys. writing to redis via Jedis cluster client).
when trying to scale and add more nodes + shared and increased the inputs we did not see any change in performance, each service is doing the same 20K operations. we would expect for any change in the ops per second for each server.

what am i missing?