With high carbon/chrome alloys, especially as-cast there can be a good deal of primary carbides and segregation of them. To get high hardness the carbon needs to get get into solution in the austenite during the soak heat treatment. This will happen as temperature is raised or when time at temperature is increased.
  It seems 980C for four hours is insufficient for the casting which you are heat treating. Raise the temerature 25C and see what improvement you get. If it is not enough, extrapolate on the basis of the change you got.
  Before you do that, however, check your microstructure to make sure you don't have excessive retained austenite, which would soften, also. If that's the case, you need less soak and possibly a lower quench temperature.

I have increased the soaking temperature to 1025C and the time to 4.5 hours.
The hardness has improved by 50 points,but not exceeded beyond 550 BHn. What else should I try? Increase the rate of cooling by adding additional blowers?

I think you may be able to increase hardness by a sub-ambient quench. Take that last sample and put it in ice water and see if you get any further hardening. If that doesn't help appreciably, then you need to further increase the soak temperature, say another 25C, to get more carbon into solution and thereby harden the martensite.