Steam boiler keeps clogging after flushing

Hi,

I recently moved into a condo with a steam boiler, and was told to flush it once a month. Our boiler is pretty new, maybe 5 years old but return pipes clogs and stops working every time I flush the boiler.

Should I stop flushing at all or it requires pipe replacement ir something? I honestly know nothing about boilers, so your suggestions would be highly appreciated.