You will only earn interest on funds which have been invested in mortgages. In some cases, there may not be enough mortgages available for your money to be invested straight away. In this case, it will be placed on the investment queue.
Once your money is invested in mortgages, interest will normally be credited to your account within the first three working days of the month. However, if your money is invested in a loan that completes on or after the 20th day of the month, then the interest due for the first month of the mortgage will not be credited until the first three days of the following month. This is to allow sufficient time for the regular payment to be set-up for the borrower.
To illustrate, if a mortgage completed on 22nd January, then no interest would be received on 1st February. Instead, interest for the 10 days in January (22nd - 31st) would be received on the first three working days in March, along with the interest earned during February.