All SWGs have different flow requirements. It could be that your Zodiac was set a little lower than the Hayward cell in terms of flow rate requirements. The SWG sits after the heat pump so while there may be sufficient flow for the HP to operate, the pressure drop across the HP may be enough that you lose flow before the cell. Also, just an FYI, but most heaters use a pressure switch to detect flow so if the pressure switch is satisfied it may not be exactly 20GPM.
You could contact Hayward and find out what their flow rate requirement is and if there's anyway to adjust it (I doubt it, but you never know). Your only other option would be to redo your plumbing around the HP with a by-pass loop to take some or all of the HP out of the flow path. Heaters tend to add a big hit to the head loss of the plumbing system, so even partially bypassing it may provide enough flow to get the SWG working.
You could also open the backend of the pump up and check to make sure there's nothing clogging up the impeller which would reduce the flow rate supplied by the pump.
