SPD 2010 workflow email based on value

I have a list that tracks the number of devices based on status by using a lookup "count" column. To this end, I know how many units I have left in inventory by the number of units with the "unassigned" status.

I would like to set up an alert that sends an email when the number of unassigned units is less than 20, so that I know to reorder more.

I'm fairly new to SharePoint, so the farthest I got is:

Step 1

If CurrentItem:Status equals Unassigned

Set Variable:availableUnits to CurrentItem:Number of Units

Step 2

If Variable:availableUnits is less than 20

Email users

then Pause for 7 days, 0 hours, 0 minutes

then Stop the workflow and log Done.

I added the pause thinking it would run only this workflow once a week. I also have it set to run when an item is added or changed. So far, I haven't been able to get my 'low stock' alert to come through. What am I doing wrong? I suspect I'm not referencing the Number of Units column correctly.