SEPA Instant transfers do not automatically fall back to standard SEPA processing. If the instant transfer fails, it must be reinitiated as a standard SEPA transfer. Some reasons may include;
(1) the beneficiary bank doesn't support SCT Inst
(2) the amount exceeds the SCT Inst limit (currently €100,000 per transaction)
(3) the payment requires additional checks
(4) the beneficiary bank's instant rails were temporarily unavailable.