Does your add-on have a date range and a rate applied to it?
The most common reason for an Add-on not displaying is that you haven’t set up a date and price for it. Each Add-on must have a date range to know when it should display. You can add a rate and date from the Add-on screen by double clicking on your chosen Add-on and inputting a date and price against it in the relevant tabs.
If you would like to hide a zero cost from showing then check the “visible but hide cost” option on the Add-on set-up page.
Are your add-ons applied to the correct units?
Another reason could be that you have only set the Add-on to display on one unit type. Open the Add-on by double clicking on it and select the relevant units on the units tab.
To update your add-ons go to: Set-up> Configuration> Add-ons.