Monitor issues

I have a customer with a dell latitude 55** using a dell WD15 docking station connected to 2 dell P2419H monitors by HDMI and miniDP to DP. Sometimes upon returning to her computer and unlocking it only 1 monitor will display, the other stays asleep. I’ve done all bios and driver updates and checked all sleep settings and tested other docks and monitors but i can’t seem to figure out a fix for the issue. Could it be the graphics card itself or maybe the motherboard? The issue happens very intermittently that i can’t really pinpoint the problem