One zone not circulating in a multi-zone system

I have a 4-zone radiant system with a high efficiency water heater as my heat source and 4 Grundfos UP(S)-15 pumps. Sometimes the system works great. Other times one or more of the zones does not circulate the water. I notice this most when two or more pumps come on at once (example if the power goes then comes on a while later or if two zones kick on at the same time as normal based on thermostat settings). All air is removed and I've confirmed its not air-bound. If I open the garden hose valve on the return side briefly (3 to 5 seconds), the zones begin to flow normally. I am not sure what to do to fix my circulation issue. Install a pump on the main return line maybe? Also, there are three settings on the pumps. I have no idea what to set these at or if they are correct since the system was installed before I bought the house recently. The radiant system is only two or three years old.