The following query help me to monitor the resize operations :
select component,oper_type,status,count(*) from (select
to_char(start_time,'dd-mon hh24:mi:ss') start_time,
to_char(end_time,'dd-mon hh24:mi:ss') end_time
v$sga_resize_ops) group by component,oper_type,status;
This query help me to determine which component oracle could not shrink or grow.
If you get too many ORA-04031 errors with ASMM enabled, i recommend you to turn it off first by setting sga_target = 0.
You should set a lower limit for each pool, so that oracle will not try to shrink it below the limit.