You will need to debug at each stage to see what’s happening. For example right before you call your update function, console.log(toUpdate) - is there the expected data in the object?
Maybe try testing with hard coded values first, then populate dynamically. Check your collection permissions.
It’s hard to say at the moment, so you will need to walk through your code step by step. If you are sure there is nothign amiss and believe it could be a bug, you can open and issue with customer care at wix.com/contact