LAX-SIN-HKT-SIN-LAX flight in about 2 months. Biz award ticket.

After a schedule change on SIN->LAX all the seats are greyed out (other 3 segments have seats selected fine). Called in, and agent said they were not yet available.

Is this just a normal issue with seat selection after a change, or is there something I should be concerned about? Is call back every few weeks a reasonable action?

TIA.