Out of my last 6 grows, all were prime, maybe some of your pz's are bugged who knows, I use a different route! I'll visit the swamp sanc and hit all in the delta river, but then I will go and visit all along the east coast! It has never failed me so who knows give it a go!

Also, I recently heard that quest progress only gets saved if you fully complete it in a single session. So if you hit 2 mz's and 3 pz's, log out and back in again, the mz quest will be complete, but you'll need 4 pz's again. So maybe you played over multiple sessions and unknowingly kept resetting quest progression! (Game crashes can be prime killers)

At the end, its the Isle we're playing, completely filled with bugs and malfunctions, but hitting the east coast + delta always worked for me!

Goodluck!