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Perhaps some crud got in the system. I would pull the solenoid body off and look for crud inside. You may have to try to use the air to blow backwards through to dislodge the crud. 

Here is a video showing the disassembly of the six packs:
https://youtu.be/MInXcfFP65Q?is=jE7IsuDhe7nTj33D

*** note it’s not good to remove the coils while they are energized, I unplugged them to make sure** it can burn out the coil if you take it off while it’s energized.

Are they making a good audible click?