There could be a number of reasons for this, but some of the most common include incorrect recipient details or exceeding transfer limits.
The current limits for GBP payments are 15 payments and £100,000 per 24 hours, 80 payments and £250,000 per week. The current limits for cross-border payments are set by our payment partners and vary over time. If a certain transfer is above the limit, you will see a message in the app with that information.
Otherwise, it's likely that the beneficiary bank has rejected the funds and so you would need to contact that bank directly for more information.