Unfortunately security screening is not disclosed and neither is it revealed what is causing the delay. Although what you can find is when the security screening started, whether CSIS is involved in the SS, and when they received the request, and if they have finished and submitted the report.
For this you will have to rely on CBSA and CSIS notes, which will have this information.
If CSIS is involved, the information obtained form CSIS will indicate when they received the request, and if they have concluded the involvement and when they submitted the report to CBSA.
To read on the SS, how many days and average SS takes and how the SS is done, see
https://bit.ly/3apWglN