Section 25A deals with unrealised rent. In this regard, no deduction (under section 24) is allowed in the year of realisation if the deduction for the same has been claimed in the earlier year under erstwhile clause x of sub-section (1) of section 24.
The reason for the above is that the deduction for the unrealised rent has already been provided in the earlier year.
However, in respect of arrear rent (covered under 25B), the 30% deduction is allowed as the same has not been accounted as income in the earlier years.