Cleaning SYR Duo DFR backwash filter with pressure reducer

Hi. We have a backwash filter SYR Duo DFR with pressure reducer in the house, old and dirty, and when you turn the ball valve to flush, no water comes out. So we installed a new backwash filter SYR Duo DFR, so far so good. But this new filter also cannot be flushed. Is the new filter defective or what could it be? Another house with the same filter works perfectly fine...