If your payment fails in this first stage, perhaps because a node along the route was offline, or did not have sufficient balance to forward the payment, it is safe to retry. However, once the HTLC is locked in along the route, the payment must be resolved. Ideally this occurs quickly, with the reveal of the preimage. However, if the receiving node does not release the preimage, or a node along the route goes offline, the payment will only be resolved after it times out. This is what is sometimes referred to as a “stuck payment”, it cannot be retried, and the sender needs to wait a long time before it can resolve the payment as failed or successful.