There could be several reasons due to which you may find credits missing in your form 26AS:
Tax deductor / collector has failed to file periodic TDS return or TDS / TCS statement;
deductee (you) had failed to provide PAN to the tax deductor / collector;
you may have provided incorrect PAN to the tax deductor / collector;
tax deductor / collector has made an error in quoting PAN of the Deductee in the TDS / TCS return;
the tax deductor / collector had missed to quote your PAN;
details of relevant challan were wrongly quoted in the TDS return by the deductor or were wrongly quoted in the challan details uploaded by the bank.
To rectify these errors in your form 26AS, you should contact tax deductor and request him:
to file the TDS return if it is still pending;
to use a PAN correction statement in the TDS return and rectify the PAN;
to furnish a revised TDS return in case the tax deductor had filed TDS return but inadvertently missed providing correct details or in case deductee had not given PAN to deductor before filing of the TDS return;
to provide a correction statement if the deductor had filed a TDS return which had error in the challan details;
to take up with the bank to rectify any mistake in the amount in the challan details uploaded by the bank.