A likely cause is that the camera is not detecting the back cover as fully closed. On cameras like the Vivitar PS 120, the motorized film load usually will not start unless the rear door latch is engaged properly.

Check these points:

• Close the back firmly and listen for a clear click.
• Make sure the latch button returns fully to its normal/top position after closing.
• Inspect the latch and door for anything preventing full closure.
• Confirm the batteries are installed correctly and have good charge.

If the door is not registering as closed, the camera may appear dead for film loading even with fresh batteries. A manual for a very similar Vivitar motorized model suggests the loading and battery steps are essentially the same, even if it is not the exact PS 120 manual.

If the latch is working and it still will not advance, the issue may be a mechanical or electrical fault in the film transport, which is common in older motorized compact film cameras.