Two things to look at -
1 - Look at the BI log and see what detection times look like, even if there's no detection.
2 - Think about adding either the dark or combined models. The dark model, actually called extra dark, is on the DS web site. The combined model is hereon IPCT.
I use the following settings for custom models in the AI tab of the camera
"objects:0,combined" for day profiles
"objects:0,combined,dark" for night profiles