There are various reasons for your demat requested to be rejected. Here are a few common reasons why they get rejected:
The physical quantity of shares/certificates received by R&T Agent from DP is less than what is mentioned in Demat Request Number or Form OR DRN confirmed for the partial/physical quantity received from DP or treated under objection for total/partial quantity by R&T Agent. Your DRF can also be rejected if the physical quantity of shares/certificates received by R&T Agent from DP is more than mentioned in Demat Request Number or Form.
All/some certificates received by R&T Agent from DP is/are found to be fake. DRN confirmed for the genuine/valid quantity received from DP or treated under objection for total/partial quantity by R&T Agent. Alternatively, all /some certificates received by R&T Agent from DP is/are reported lost or stolen and a stop is recorded.
Duplicate certificates earlier issued by R&T Agent in lieu of all/some certificates received for demat by R&T Agent from DP. Alternatively, all/some certificates received by R&T Agent are found to bear forged or fake endorsements of Name(s) of Holders.
Name(s) of Holder(s) on all/ some certificates received by R&T Agent differs in Demat Request Form received by R&T Agent from DP. OR DRN confirmed for the valid quantity received from DP or treated under objection for total/partial quantity by R&T Agent. Signature(s) of Client on Demat Request Form does not tally with a specimen of signatures Holder(s) recorded with R&T Agent.
Stop recorded as per Bank lien/Statutory Authority/Court Order/etc. in computer master file(s) of R&T Agent against all/some certificates) received for dematerialization from DP.
There are many more technical reasons for the rejection of your demat request and you can refer to the SEBI website and the NSDL/CDSL website for details. These are the popular reasons why the demat request gets rejected.