Normally the employer has to obtain from his employees, the proposed investments he will be making during th year. On the basis of these investments provided TDS is required to be made. However if the taxable income is below the exemption then no tds needs to be made.
However at the year end say in dec, the employer needs to obtain the actual proofs of investments from his employees & then TDS needs to be made. So if TDS was not being made from the beginning & is required to be made after submitting the actual proofs, then there is no default on part of the employer.