Possible reasons:
The reasons for not showing the credits in Form 26AS are due to TDS/TCS statement may not be submitted by deductor or collector, deductor or collector may not be provided with PAN by Tax payers, deductor or collector may be provided with wrong PAN by Tax payers, in TDS/TCS return the deductor or collector may provide an error to refer PAN of Tax payer. The reasons may be due to the wrong reference of PAN by the deductor or collector. The challan details in return of the deposition of TDS/TCS statement may be wrongly cited in statement by the deductor or the bank cited wrong details of challan.
Rectification:
In order to rectify faults the deductor must file those TDS/TCS statement if it is not done. If PAN is error then in the TDS/TCS statement PAN should be rectified by using PAN correction statement. Before the deductor filed the TDS/TCS return the TAX payer must submit PAN correction statement in case the deductor forget to provide the details due to lack of care. If the TDS/TCS statement contains the error in challan details then tax payer must submit the correction statement for rectifying the error. If the challan details contain any mistake in the amount then tax payer must consult with the banks.