Recently published paper deliberately not citing our very relevant work -- anything to do?
You have no real recourse. Authors can cite what they like and snub others for no reason. Plus, you can't cite everything so someone's going to get left out. But if it seems purposeful it very well may be. I would send your paper(s) to the snubbing PI and say something like 'looks like these slipped past you'. 

It's in their best interest because of your work truly is that related, you'll probably review their papers eventually.