Problem solved! A few items in a shopping list do not synchronize. All others do.

This is how you solve the problem:

• create a new shopping list
• open old shopping list
• Click on Menu - select all
• click on copy icon and select newly created shopping list

Have all items been copied and appear on all connected devices? Then you can delete the old list. The problem seems to occur when items are deleted and added to a list very often. The cause has not yet been found.