Xero limits the number of requests that your connection can make each minute, and each day. These limits are put in place by Xero and are outside of our control.
These limits are good because they ensure a high quality of service for everybody, however it can mean that your data can take a little longer to come down.
The most common cause of hitting this issue is compounding API calls.
The key thing to note here is that the limit is based on the number of API requests - not necessarily the amount of data you are bringing down - an API request that returns one result contributes just as much to your usage as a request that returns 100 results.