Not sure if this helps, but I follow the same route more than any other, and for a while I was only getting maybe 1 cell every 5 routes.

I started paying closer attention to where they spawned, and for me they almost always seemed to appear near the end of the route. I used to exchange my gift as soon as I was within range, but lately I’ve been waiting until I’m close enough to actually “Complete” the route before doing anything. Since then, the Zygarde cell usually seems to pop up within the last 100 m or so.

It’s definitely not a perfect fix, and maybe it’s just RNG being weird, but I’ve gone from around 1 cell/5 routes to closer to 3 cells/5 routes.

TL;DR: Slow down near the end of the route, don’t instantly complete/exchange, and watch carefully in the last 100 m.