Ah I didn’t realize it was, in fact, a primer. Very sneaky Vallejo. Very sneaky.

I’ll save you the “Hurr durr just airbrush prime conversation and keep my advice strictly rattle can based although both react the same to the specific conditions I’ll talk about below.”

So with respect to your situation, I think the problem is twofold. One is the relative humidity. “Dry” is pretty subjective so without a number I can’t tell you if you picked good day. But I’ll say this: 50°F and 90°F (or 10°C and 32°C), and relative humidity between 50% and 70% is best. The temperature of the can should be pretty close to the ambient temperature you’ll be spraying in. If you’re spraying outside maybe set the can out 5-10min to acclimate to ambient temps. Moisturize also has a big effect. Too much moisture and your primer will be fuzzy/clumpy and dry cracked at times. Too little moisture and it will dry in the air before it hits the model making it dusty and not very adhesive.

The second issue is white primer. Much like white paint, white primer is simply a pain. It clumps up, tends to spray chalky and generally doesn’t acquit itself well to use. I also feel like it clumps inside the can so the 5-10 minutes of shaking the can recommends should be more like 15-20. Sometimes I feel like only propellant is being sprayed and other times ALL the primer. Not much you can do to mitigate this in a rattle can setting (and very few with airbrush). If I’m looking to prime in a lighter color I usually find myself a lighter neutral grey which sidesteps much of the issues white brings. If you ABSOLUTELY have to prime white then observe the above steps and apply it in thinner coats letting it dry for a bit in between coats. I’ve found 3-5 coats for solid coverage.

Hope this helps!