When you Zap liquidity, it normally requires multiple transactions (unless zapping ETH), which you'll be prompted to confirm. However, transactions on Ethereum compete to be confirmed through an auction-like process, bidding by gas price (shown as Gwei in your wallet). If the average bidding gas price is much higher than you selected under Speed before Confirming a Zap (see screenshot below) or when you modified the gas price in MetaMask, your transaction can get stuck. Sometimes, a Zap user can get unlucky choosing a gas price right as Ethereum gas prices skyrocket. It's a best practice and recommended to check gas prices at ethgasstation.info before clicking Confirm.

Stuck Transaction

Transactions submitted by an Ethereum address will processed sequentially so if transaction #1 is stuck, transaction #2 will be stuck waiting until #1 confirms. Here's a helpful video tutorial from DeFi Dad on how you fix a pending transaction (a.k.a. unstuck a stuck transaction).