DB log buffer not increasing

log_buffer does not get updated after an oracle instance cycle.

i'm trying to increase the log buffer size however the database is not taking the change.

i'm used this syntax

alter system set LOG_BUFFER = 8388608 scope=spfile sid='*';

This appeared to take w/o error and i cycled the database.

Upon cycling the database i find out that the log_buffer setting is back to being 8355840.

i need help on how to fix this

11.2.0.4 Enterprise Edition

RAC, ASM, AIX7100-03-04-1441

Thanks