Amazon does not always accept dimensions entered through List & Prep or Seller Central in a timely manner.
If you have entered dimensions and they are not being accepted, you are able to submit your dimensions in a different way that is not at the mercy of the Selling Partner API (the system 3rd parties use to communicate with Amazon).
If you have entered dimensions through List and they are not being accepted, here's what to do.
Add the Item to Your FBA Inventory
If the item does not yet exist in your FBA inventory, you'll need to add it and save it in Seller Central. To do this, click the hamburger menu (three stacked lines) in the upper-left corner of your Seller Central screen.
Go to Inventory > FBA Inventory.
Click Add Products in the upper left.
There you can search for your item and add it to your FBA inventory to generate its MSKU.
You now have two options:
Option 1: Add Dimensions in Send to Amazon
Navigate to Send to Amazon. This can be done from the FBA Inventory screen by clicking Shipments > Send to Amazon in the upper left.
Add the MSKU to a Send to Amazon workflow. An easy way to do this is to click Start new in the upper right and then search for your item. When it appears, click on the Data is missing... message under Information/action.
Enter the dimensions, then click Save.
You can now safely delete the Send to Amazon workflow in Seller Central. Return to InventoryLab and list the item as normal.
Option 2: Add Dimensions in FBA Inventory
Return to the hamburger menu > Inventory > FBA Inventory in Seller Central if you are not there already. Find your item (you may have to search for it). Then scroll all the way to the far right of the screen and click Edit listing.
Click Continue in the popover that appears.
This will take you to a new tab where Amazon will ask for all required info not yet supplied for your item. You'll find the option to submit the package dimensions and weight under the Offer tab.
Fill in the missing details. Finally, scroll all the way down and click Save and Finish at the bottom right.
Be sure to copy the Seller SKU (MSKU) from Seller Central if you want to use it for your listing in InventoryLab. Return to InventoryLab and attempt to list the item again.
Additional Troubleshooting
If the dimensions still aren't being accepted, try opening the Seller Central pages in a different browser and try again. If it still isn't accepted, reach out to Amazon Seller Support's FBA team for assistance.