Nest spawns randomly being disabled/not spawning?

I think I have noticed this happen before once or twice...in previous nest migrations, and im pretty sure its happening now (in my area at-least, dunno if its world wide or what)

But...the nest pokemon are not spawning at my local park....they was around for a few days then gone.

It changed to Growlithe in the last migration, he was popping up everywhere throughout the park as the nest spawns usually do (its a massive park)

But now there's none of them, I didn't even realise at first that growlithe just seemed to have stopped spawning completely,just a few days after the migration. I already have tons of candy so I was not bothered about farming him too much or anyway, so i didn't really notice it right away that I had not seen a single one in the park for days.

but I realised when I went to another nest the other day, (must be several miles away from the Growlithe nest) just a smallish park, that was marked as a Machop nest..and I had no luck seeing any Machop at all either.

This is what made me think that the nest spawns are randomly being disabled, or at least some of them perhaps.

Im sure i have definitely seen this before, where theres just a period where the park just seems to have no nest. I think ive noticed it before around event times, maybe after/during increased spawns such as more fire pokemon etc they just seem to get disabled.

How are other people finding nests ? are you actually getting the nest spawns spawning ?

have you ever noticed periods where a nest just seemed to be inactive ?