The string may not actually be in the cocked position, resting on the string latch. It may instead be resting on the DFI (Dry-Fire-Inhibitor) lever located in the front of the trigger box.
When the trigger box securely latches to the bowstring, the safety knob will move from “F” (FIRE) to “S” (SAFE). You should see this movement and hear a “click” when you push down on the charging handle to secure the trigger box to the bowstring. If you skipped this step in the cocking process, the bowstring is likely not engaged with the string latch and is being held by the DFI.
To correct this condition, please view the video How To Free the Bowstring from the DFI on ACUslide Series Crossbows or follow the step-by-step instructions below.