Since September 2022, Particle has moved its project over from the Avalanche chain to the Ethereum Blockchain.
If you minted or bought Particle of LIITA before then, it is most likely that they are on the Avalanche chain and you cannot see them on the new platform. You can check that you still have access to them by connecting your wallet to an Avalanche third party marketplace such as Kalao or NFTrade.
As of September, we have built a bridge that allows you to transfer your Particles of Banksy's LIITA from Avalanche to Ethereum. You can use the bridge here, and access more information and a step by step guide on how to use it here.