Buderus Radiators

Hi guys. This is my first time using this forum, so apologies if this topic already covered elsewhere. I'm a homeowner who recently purchased a 100 + year old home in North East. It has hot water cast iron radiators and oil boiler. I had a new Buderus boiler installed last year (G115WS). System worked great until we tried, as part of a house reno, to replace several of the cast radiators with Buderus Model 21's. We cannot get them to heat up despite rest of the system working. Flushed them a bunch of times. The system is a multiple one pipe system with home run supply and return lines to old pipe loops in basement. Heating lines to all radiators are new 3/4" PEX. We installed 1/2" compression fittings on the Model 21's per the bosch specs for the units. I purchased thermostatic sensors but not installed yet. The circulator pump is whatever comes with the G115WS and is turned to the highest setting. I spoke to the vendor (supplyhouse.com) who speculates that I need a higher powered circulator pump but that makes no sense to me as the boiler is new. Any help you can provide would be much appreciated.

many thanks.