Just had a similar problem when dewinterizing. I did not purge the outside shower because we are still getting temps in the mid 20s or lower at night some and I wanted to leave the antifreeze in the lines. My pump worked fine purging lines until I opened the line to the water heater. It took more time than expected, but then after the water heater was full, I could not get decent flow... I ended up purging the outside shower lines and once all the air was out, the system pressurized fine and has proper flow.
Do you have all the air purged from the system? Is your valve for putting in antifreeze completely switched over to the fresh water tank?