Is there an error message that prompts you once it fails to add a custom model addition to your collection? If so, what does it say? How many do you have in your collection right now? Have you cleared your cache/cookies? Which device/browser are you using? Ex. Mobile, desktop, PC/Mac; Chrome, Firefox, Edge, IE, etc.
The more details you can provide us the better we can narrow down the source of the problem and have a fix for it sooner rather than later. Thanks for your patience and understanding.