what do you consider a lot of shaking, IF I have use for a rattle can, I shake it for three of four minutes, then I warm it in a bath of very warm water. THen I shake it more,, and more, then heat, then shake again. I would be leary that the nozzle is clogged or messed up, like someone started to use or spray it, then did not invert and clean it.
also keep in mind that primer is primer primarily because of the particulate matter in the product. Keeping that in mind, if that can sat for months or years, it would not be a surprise at all that it would need a LOT of shaking. I mean 10 to 15 minutes coupled with hot water bath. It can settle out very hard.