error in db2start v8.

good morning,
I have DB2 V8 32-bit AIX 5.3.
I have run this to change the memory of the instance:

db2 update dbm cfg using INSTANCE_MEMORY 262,144

and running the db2start command gives me this error:

11/01/2012 00:00:24 0 0 SQL1220N The database manager shared memory can not be September allocated.
SQL1032N No start database manager command was issued. SQLSTATE = 57019

can someone give me a hint of why the error?

Thank you very much and best regards.