Hi Aakash,
Since you have created a new custom task, so you have to adjust the UWL XML configuration on portal. You have to take assistance of Portal consultant.
Download UWL XML and add a new entry for custom task ( Just copy the TS40007953 UWL XML and just replace the TS task no) and re-register the system and clear the cache.
How to Upload an XML file to the Universal Worklist - Business Process Expert - SCN Wiki
This will resolve your issue. Take help of Portal consultant to ensure UWL settings are correct.
Regards
Sandy