system drained, heat not circulating

[jer850]

I drained my heating system (forced hot water, heated by gas) to replace some piping and since turning it back on the heat is not circulating through the system fully. It's in 3 zones and only 1 zone is fully hot. The feed piping before the area I replaced is hot, but the piping beyond that is cold, which I believe is the return pipe . Do you think this could be air in the system, or something else? I did drain quite a bit of air from 1 of the radiators, the others spit water as soon as the valve was opened. I used Shark Bite connectors in the area I repaired and am wondering if these could be restricting the water flow. Before I drained the system, it seemed to be working fine. Not sure if there's another way to drain air from the system, which is city fed closed system.
Thanks for any help.